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.