I would need a fair amount of mortar if I went this route but
presumably it would be far cheeper that building a stud frame
& then insulate/plasterboard/plaster?

But building a stud partition is very quick and almost free by most DIY
standards. You'll need about 10 3m lengths of 75x50 and 6 sheets of
plasterboard. You'll also need some nails. That lot will be about 50 quid.
The expensive bit is doing the door, frame and furniture, which you'll have
to do with the brick wall anyway. I suppose you'll need some way of getting
the plasterboard home, though. My minibus with roof rails means I never need
to think about such things.

Plasterboarding is much simpler than the wet plastering you're intending to
do on the brick wall. There's not normally a need to insulate an internal
wall, although it can be beneficial in some circumstances.

Christian.