After a lot of reflection I've decided to go with the Canon duplexing mono printer; at an effective price of £50 after cashback it works out at about 2.5p per page with the starter cartridge, 2p a page with high capacity cartridges. The last colour laser I looked at was very tempting, but in the end I think I have to accept that colour isn't something I need very often; I may buy myself a cheap inkjet for occasional colour work, I haven't decided yet.
The bottom line, I think, is that if this turns out to be a bad move I'm only out £50 (and possibly the price of an inkjet later), and have numerous family members who could make use of e.g. the mono laser if I do eventually decide I need colour for my main printer.
Later - looking at cheap inkjets, Staples are doing the HP deskjet 1000 for about £20 inc. VAT. Anyone know anything about them?