xmodmap. I immediately had a poke around, discovered it was a standard tool included with X, and upon reading man page found that it was exactly what I was looking for!
So now, courtesy of the command
xmodmap -e "keycode 113 = Return", Alt-Gr does the job of the Enter key. I have a usable system once more!
But although it's great as an interim measure, I can't live with it permanently. Once my dad gets back from Austria I'll investigate getting it fixed properly. Bet it'll be expensive.