Change network and keep the same number?

First, check that the handset he is using is not tied to the EE network. Here's a link to Ofcom, which explains it much better than me
www.ofcom.org.uk/phones-telecoms-and-internet/advice-for-consumers/advice/mobile-phone-locking-and-unlocking

Here's a link to the step-by-step process to change the number
www.uswitch.com/mobiles/guides/porting-your-mobile-phone-number/

It's very easy to change the new SIM to the old number

Where we are we can only use Vodafone, we don't get a signal from anyone else. Very frustrating when we have visitors on other networks!

You can keep the same number, follow links above. If not, to find the new number just use your fathers phone to phone yourself and you will then have the new number.

Thanks both - I seem to have a mental block about sim cards! I think I must have got a child to do it in the past

Goodluck Eddiecat and let us know how you get on

Very simple to keep the same number. I've had the same number for many years including switching networks and several phones. Advice to use uswitch is good.