I'm currently trying to edit a live event and sync up the two cameras so I can do a multi-cam edit, but one of them seems to gradually speed up slightly (like it shows 2 frames before the other one), like it's 24fps or something, but it Premiere says it 25fps.

I try and re-sync up, and it just eventually starts to slip out of sync, any ideas?

Ah, I think it's because one was progressive and the other was interlaced for some reason and Premiere is reading it as 25fps because it is just interlace with 2 fields.

I'm guessing the best option would be to convert the 50i one to 25p? What's best way of doing this?

You could do a batch conversion in MPEG streamclip or put all the 50i footage on your edit timeline and export it as a 25p version.