MaggieP:I want to keep my mobile number but change from pay monthy to pay as you go - is this easy?
That's a great question. The answer is it should be easy but it can be fraught and you need to keep focussed.
If you want to stick with your current network on PAYG ask them. Probably they will try to offer you incentives to stay on contract, sorry flog you a new minimum term. There is a really important often misunderstood concept here, contracts don't normally "end" only the minimum term expires unless you stupidly agree another minimum term.
If you want to move your number to a different network, you simply ask your current network for a PAC. Again expect to be offerred incentives to stay. Up to you to manage that. If you pass that test you should be sent your PAC. You have 30 days to transfer. Then you need a new PAYG SIM for your new network and an unlocked phone or a phone for that network. When you receive your PAC you phone your new network and ask for the number to be transferred.
Once your number is transferred your contract with your old network is teminated. You may have to pay for a 30 days notice period. Once you are satisfied your contract is terminated and fully paid off remember to cancel your Direct Debit.
There is an industry mechanism for all this but you need to keep your focus to avoid being "conned" into a new minimum term when you set out to go PAYG. The easiest number transfer I know is ASDA because their leaflet offers clear advice.