We are in the process of redecorating our back bedroom and the steamer
is making hard work of getting the wallpaper off the wall which is
unplastered plasterboard.

I find slashing the paper with a Stanley knife before steaming is effective
at getting the steam behind the paper. Unfortunately, it is as likely that
any technique is going to be as effective removing the paper face of the
plasterboard as any subsequently applied wallpaper. You may find it easier
to simply remove and replace the plasterboard at a few quid a sheet and a
few more for filler.

Christian.

Thanks Christian, is it technically difficult for a newbie DIYer to
replace the plasterboard?

Assuming it is plasterboard on a stud partition, then it is extremely
simple. It just requires enough strength or a helper to manoevure the board
into position and a good battery drill to knock in the plasterboard screws.
Dot'n'dab drylining is harder to replace, though. Use "taper edge" board and
then tape and fill the joins. Because the tapered depression is narrower
than the float, it is easy to get a flat surface with a bit of polishing. It
is nowhere near as difficult as real plastering.

Christian.