I was thinking of changing from Vodafone PAYG to Vodafone contract and changing my phone... not exactly sure which price plan to go with yet though...

Does anyone know if Vodafone will let me keep my current mobile phone number if I change from PAYG to contract?

Yes, they should do.

Thought I'd add:

Play it right with the Retentions department (say your going onto o2 Contract etc etc) and they'll almost certainly give you some kind of discount (or free handset) on one of their contract deals...has worked a gem for me in the past!

Lee