Mind her own...business...

GrannyOrNanny, in regard to your query, You are obliged to work the hours set out in your employment contract terms. Your contract may also state something specific in regard to overtime, For example, very many employment contracts state that "a reasonable amount of overtime may from time to time be required, in accordance with the needs of the business".

The above terms are very much included in employment contracts drawn up in the Transport Industry, retail services, parcel home delivery or any business where customer demand can be volatile for any number of reasons.

The best practice for any employee is always to discuss such matters with their direct line manager. As you state GrannyOrNanny in your opening post that the situation has only come about due to the loss of an employee on a different team, then it may well be that a temporary solution is required by your employer while a new employee is recruited.

Reference to overtime working can be found by following this link:-
worksmart.org.uk/work-rights/pay-and-contracts/hours-work/my-employer-wants-me-work-extra-hours-excess-those-set-out

If you don't want to do it do'nt.
If you do it once you will be stuck with it, as everyone will assume you can always provide cover.
You would be making a rod for your own back.

I wonder if your colleague who works "nearly full time" is concerned that she may be asked to up her hours and now go full time to cover the work - possibly doesn't like the thought either and therefore is suggesting you work the Fridays. On the other hand may have no ulterior motive at all. I would wait until management decide what they are going to do about it, and if you are asked if you are interested say no if you don't want it.

YANBU Take it you are not responsible for the staffing in the place. Who ever is should be addressing the shortfall in the other team certainly not you.

I find it very hard to no!
Before I got ill someone left without working notice.
I was asked would I cover until they got someone else
Yes I said as long as it isn’t too long
3 months later I was still doing the extra hours!
7 days a week 8.30-5.30...
When I mentioned it several times I was told they haven’t found anyone suitable
One day I completely lost my temper and said if it isn’t sorted by the end of the week,your be looking for 2 new employees
So stick to your guns and don’t be a mug like me

Perhaps your colleague was just trying to be helpful. Maybe she thought that you would welcome the extra day’s work and extra pay. As you are happy with the status quo just let it lie, unless your manager approaches you with the offer. Then you can politely refuse. You don’t need to give a reason, just a ‘thank you for offering it to me.’

Hope the responses you have received have helped you. GrannyorNanny, they mostly seem to be singing from the same hymn sheet. When I was working part time (many moons ago ☺️)
I used to get into the lift at 1pm to go home and one of my colleagues would be there going for her lunch every day without fail she would say to me must be lovely to work partime and be going home, and I would reply everyday Oh it is you should try it .
I think some workers not all by any means do feel a bit of resentment to P/T workers.
Also remember the old saying no good deed goes unpunished.

Stick to your guns pet

I worked in a busy office, and one colleague left abruptly - to put it mildly. She just upped and stormed out. We were in a fix, and I was asked to help out, which I willingly did for quite a while, putting myself under stress as a family member was in hospital. A kind colleague took me to one side and said that I had done my share, and it wasn't MY responsiblilty to cover for absences in the office. It was a light bulb moment - I went to see the manager and said was he making any effort to get a replacement? No, he wasn't! I said that I wasn't going to continue with the impossible workload further than the end of the week - and hey presto, a replacement arrived from a temp agency on Thursday. The point I am trying to make here is that we perhaps all feel a duty, a 'Protestant work ethic' but in truth it is not up to you to cover for colleagues more than if someone goes home ill and is likely to be back in a couple of days.

Please tell your colleague that with respect your hours and conditions of employment are none of their business.
Simple as that, no more discussion or conversation to be had.

Why has she taken it upon herself to ask you to do extra hours when it’s nothing to do with her, it’s up to your employers to either approach you themselves or replace whoever has left.
I would just say to her you don’t want to do it and leave it at that.

Just say it's not possible. There's no point undertaking additional duties if you are going to resent them.

Why did she ask you, of all the others. If she works "nearly" full time, she's got time to work... so why doesn't she do it herself? You went in to that position with a job description. i.e. Fridays are your off days. You are not obliged to take up her suggestion. She wants to look good with Management because she'd have "found someone" to do the job. She didn't even mention tat your paycheck would be increased.

Your colleague would not be in a position to authorize payment for you if you did extra hours so is she expecting you to help-out for nothing? I experienced a lot of jealousy at work because of my part time hours but I would then reply that they wouldn't want my part time money!

Do you mean you have been asked to work an extra day? If you dont want to say no. You are under no obligation to do it.
As for people being jealous because you work part time. They shouldn't you work part time and get part time money. They work full time and get more salary. I used to work part time and it used to drive me crazy that people used to expect me to have a full time workload and do as much as someone working full time when I was being paid for part time!

Granny or Nanny
I personally, at times, in my working life felt used and ceased being 'available'. Why should it be you who fills a gap ?.My attitude will without a doubt to some appear selfish, but I would want to know' What's in it for me'?

Could you offer to do the Friday slot for say a couple of months until someone else is found to fill it. Say you don't want it to be permanent but will do it till Easter (for instance).

Madmaggie you are so right about the backbiting that goes on with full timers thinking that part timers should be doing more work! I kept having to point out that I earned half their wages but they refused to see it!

Sorry, I missed that she is your colleague not the manager.
It is not up to her.

They need to recruit someone else as one person is not a team!

Perhaps she thought she should offer you the opportunity first rather than you think 'why did she ask them and not me'.
Presumably you would be paid in full for the extra hours.

Perhaps she knows you will do a better job.

If you don't want to do it just say 'thank you for the opportunity, but I have other commitments and cannot do more than the hours I do at present'.

Grannyornanny I've been in similar situation myself when I was working. With the benefit of hindsight I would suggest - put yourself first because no one else is going to, in fact sounds like they're happy to take advantage of your good nature and that is usually the thin end of the wedge work wise. Work one or two Fridays 'to help out' and before you know it, it's become the norm. Having Fridays as your day off was always jealously guarded where I worked as everyone wanted it so it made a long weekend. And the backbiting was awful. If your colleague can't cope it's management's problem not yours - although I'm sure they'd happily 'permit' you to solve it for them!! Assert yourself or, throw in the towel. You do not have to explain to anyone how you spend your Fridays, it's your day off - end of. Wishing you well.

It's not your colleague's place to even suggest it. You work part time - on the days that you're available.

Susan56 is spot on.

YANBU. Definitely not. It is not your problem. Tell your colleague you don't want to work Fridays and if approached by management don't make excuses that you have other comnitnents. Just be assertive and say no thank you. No need to say sorry.