Spent the day arguing with myself about sticking with 7 speed or going to 9 and being all modern and trendy. This involved a huge amount of looking at online shops. Got the 9 speed cassette ready to go, put it on the wheel, looked at it for a while.
It all seems a bit too tricksy and narrow for easy set up and maintenance and would mean buying a raft of new bits when I have everything to stick at 7 apart from a £3 spacer so it works on the wheel I want to use. And I might have a spacer the right size anyway.
Secret Seven all the way then.