I am planning on using 50mm Celotex between the rafters. Due to cost,
I was planning on using 25mm polysterene from B&Q over.

Technically, the top layer of insulation has to be better or the same than
the bottom layer of insulation. That means that if you have 50mm Celotex
between the rafters, you can have either have up to 50mm below the rafters,
or 50mm or more above the rafters. The top layer being polystyrene makes it
even worse.

This is to avoid condensation forming between the insulation layers.

If you are going to suck it and see, then DEFINITELY use vapour check
plasterboard, as that will significantly reduce the problem. Taping the
celotex with foil tape would, I presume, have a similar function to vapour
check p/b.

Christian.