I have a tiled roof and felt on the side facing into the loft. What is the best way to insulate inbetween rafters? Do I need to leave a 50mm vapour gap?

Yes you will need to leave a 50mm void.The best insulation to use would be celotex or kingspan insulation board.This is easy to cut and should be cut to a snug fit between rafters.You will need to be careful when measuring for your cutting though as rafters do not always run parallel and you do not want any gaps or boards that do not fit tight enough to stay in place.

Exactly as Roc has said, and yes make sure there is minimum 50mm air space.