<SOLVED>
Hi, I've fixed my problem, but it involved a reformat.
I tried the suggestions as advised by Saint, and got a copy of the latest version of RockBox.
I then selected 'failback image' from the tools menu, dosconnected, etc - but although windows recognises the device as a "removable drive", I still couldn't access it.
I was able to startup into UMSBoot partition and access that, so tried dropping the .ubi file there (
http://www.freemyipod.org/wiki/EmCORE_Releases).
It flashed ok but made no difference to my original problem.
So from the tools menu, I selected the 'reformat partition' option, restarted and suddenly my device is accessible again and shows up as 'iPodClassic' in win explorer.
I've extracted the latest version of rockbox to root and am now copying my music files over.
Looking forward to checking it all out on my lengthy train trip in to work tomorrow.
Thanks guys for pointing me in the right direction.