Thanks for the responses.
I was checking that all drivers etc. were up to date and what I noticed was that I had two folders both called "My Music". One contained all of my music whilst the other contained a few MP3 files which I had recently downloded from Amazon.
It seems that Windows 7 automatically created a new "My Music" folder to hold the downloaded files from Amazon and it was this folder that Windows Media Centre was going to for its files. Finding nothing in it, WMC then looked at other locations and rebuilt a library of music which it promptly lost when the machine was powered off.
I also notivced that when I deleted this folder, W7 automatically recreated it.
I copied all of my music from "my" My Music folder into the W7 My Music folder and, hey presto, WMC now works
Closer inspection also reveiled 2 "My Pictures" folders which I also sorted out in the same way.
I suspect that the problem is due to the fact that I upgraded from Vista to W7 and, even though there were existing My Music and My Picture folders, the upgrade created duplicates which it regarded as the master folders.
Thanks again
B-E