It turns out that the plastic that the screw goes in to to push it against the pins underneath had snapped. No idea when this happened, but there are also missing screws. I've got some replacement screws and a replacement housing set off ebay (fake I'm sure, but it has the aerial with it). At present, a piece of blutac underneath the battery cover is doing the job just fine. That post has saved my N900! I've spent about £75 repairing in, but that's cheaper than a new phone and a new contract. Plus, I want to keep my N900! :-)

Thanks to both of you.

Steve