Unfortunately it sound like its too late for you.
As horatio says, switching on the sidelights or an interior light etc can help prevent this. It is the "voltage" spike that occurs when the battery is reconnected that causes the trip computer display to go.
By having something else switched on, that draws the current and will "absorb" the spike.
Sorry, we should have been more specific in the original post. The concern is that the trip computer reads miles since fillup, avg mph, miles to service, distance covered on current trip/total covered, but not avg mpg, fuel amount used since reset or miles left in tank. Consumption reads in mpg not kpg, and distances feature in miles not km. Your further pontifications are welcome; thanks for the help thus far.
No worries. It's just that the 'gone into km' mode problem seems to be the most common one, especially after the battery has been disconnected.
This one's not unheard of though, and I had it happen on a Lag I once. The dealer told me it needed a new instrument panel, but the next owner got it sorted by another dealer for their minimum diagnostic and labour charge.
If you're very lucky, it might just clear if you do a dash diagnostic, though I think when I last suggested that, it didn't work. Anyway, nothing ventured, nothing gained.
Hold down the button on the end of the wiper stalk while inserting the keycard. The dials on the dash will start to step up and down and most of the warning lights will come on.
Step through with the wiper stalk button until you see either a series of four dashes or a mixture of dashes and letters on the mileage display.
Press the trip reset button on the dash, and the dash should go back to normal.
If you're very lucky, your fuel functions will have come back. You'll probably have to drive the car for a mile or so before knowing for sure though. If not, you'll have to find a dealer or Renault specialist who'll agree that they can sort it without replacing the whole panel.
Some of the features of the trip computer may not be available on the UK/Miles computers. On my Avanitme I have average mpg but not current mpg. Also it doesn't have total fuel used. This is only available on the metric display I understand.
Thanks for that gents; Toddy is off back home but I'm viewing this on his behalf; he's not got a pc and I do all the typing for him ;-) No Hugh, he wants the units in miles not km but I get your point; we had a suspicion that might have something to do with it, and Horatio that was very helpful; I'll feed it on and report back when he's tried it. Many thanks.
David tried the wiper stalk reset thing and the dash lighs etc lit up as indicated but no change was found to the trip computer functions. He's still missing the ones earlier advised; could it be a case of the fact that these functions are simply not available for a UK spec car? David can't remember whether they were pre-existing, but has read of them in the manual and understandably would like the functions there if they should be. Many thanks again.