An interesting and curious thing (well, to me).
I suddenly remembered that I have another (sim-broken) n900 in a draw, and so charged it up to see if that one could use the ovi store. Sure enough, it could download and install applications without any difficulty!

The curious thing is that it has exactly the same firmware and repositories as my 'working' n900...
It also still has a working facebook widget!

So, am I right in thinking that I can use this one to download the debs, using
'apt-get install -d' and then copy the deb files over to the other n900?

And, thank you quailstorm for the apps.. Much appreciated