I've used them for 2 mobile phones over the last 2 years now. Had no issue.

I used them for the very reason you are stating, Vodafone just couldn't match the price at all. I had no problem switching my old number (EE) to my new Vodafone account.

I believe vodafone will transfer numbers if you are already a customer of theirs without a PAC. The problem is, generally when you go through third party sites like mobiles.co.uk, they will set you up as an entirely new customer which means you would need a PAC.

This forum on vodafones site seems to suggest its a huge pain in the arse, if I'm honest. Basically it looks like you have to transfer your old/current account to PAYG, essentially meaning you've kept your current number on a pay as you go account you don't actually want or need, wait upto 30days for the number to transfer and then you can go ahead and transfer it to your new account.