Graeme brought next idea :
Just found an unopened packet of wallpaper paste, that must be 20 years old.
Does this stuff go 'off' or will it be OK to use? I don't want to mix and
use it, then find paper doesn't stick well, given that it is hardly expensive
stuff.

We have just done (nearly) a top to bottom redecoration with some paste
of similar age, no problems noted.