Just fitted a loft hatch on Saturday, one off ebay - about £70. Polish made. Optistep make. Went in ok. Fitted 3 others - all Youngmans and to be honest I'd stick to Youngmans because of a better hinge mechanism. I intend to do as others have done - insulated plug in the loft space with a draft excluder no further minimise drafts (one good thing about the Optistep is that its the springs (very strong) that 'close' the hatch so there's no air leakage from an opening mechanism).
The 55cm wide ones should fit into joists without cutting any timbers, I've fitted the 60cm one so did have to modify the roof joists. All pretty straight forward though. Settled on a 110 cm length, 100cm is ok, much shorter than this and getting in and out with 'stuff' is a pain
Cc