Thanks for all of your replies ladies. They were very comforting at a very stressful time. I learnt about things I never knew existed (breast warmers and baby-led attachment!)

I'm thinking it's actually thrush, so treating with daktarin gel atm, and if it's not cleared up in a week or so I'll head off to the GP. Will keep expressing for the time being - at least he's getting fed one way or another!!