Thank you all for your comments, it has been very useful to hear what others think, even some employers 
It is a very small organisation with only two part time employees and is almost run like a not for profit company and like a very small family business. I have a very good relationship with the people who work and run the business; it’s also a bit like a co-operative.
I’m not a member of a union and generally very happy working there, this request to change the sick pay conditions came totally out of the blue but is in relation to a general overview of the two job descriptions and contracts.
I do understand that the sick pay is very generous but it was part of my original contract 7 years ago and very comparable to previous employment over the last 15 years or so. I’m also aware that I am more likely than ever before to have need of it in the future due to getting older and health problems are more likely to crop up.
There has been no ‘sweetner’ offered just a drastic reduction in the sick pay allowance. Also my contract states that I should receive a ‘cost of living’ rise annually, which I have not always received, sometimes due to the financial status of the company (not dire, just large outgoings at the time) or an increase in hours at a similar time to the salary increase date and I have not pursued it at the time.
I think what I might suggest is halving my current allowance to 3 months full pay and 3 months half pay, but that they also increase my hourly rate to what it should be if I had had a cost of living rise every year since I’ve been there. I wouldn’t be asking for back pay. How does that sound to everyone – is it a reasonable request? 
Thanks and best wishes