Thanks - the battery was actually new in September 2012, so ought to be ok (but may be faulty of course).
I'll try disconnecting the battery tomorrow for 5 minutes as you suggest (too cold to be messing about outside at this hour!). Thanks for your help. I'll let you know how I get on.
Just to let you know, I disconnected the battery for 5-10 minutes and then reconnected. Was pleased that immediately a mileage showed on the odo and then, on firing up, I saw all the instruments were in business again. Thanks for your advice - sorry it seemed to be such a trivial issue! I should have realised the first thing to try with computer problems is to "turn it and turn it on again"!