Thanks for your replies. With help from Dell technical support, I think it has been resolved.
Booting using the installation disc, we tried to reinstall the OS, but there were no disc found. We had to install the drivers for the raid adapter using a USB drive.
It appears that the drivers for the raid were not installed. Booting again to the recovery console, and installing the raid drivers, the drive was then found using DISKPART.
Then using the information from here, I was able to get the system to load the O/S
support.microsoft.com/.../2501318
Not sure if this was down to me messing about and changing the active partition. I still don't think I did change anything. A windows update that had been installed and was awaiting a restart, or an issue with the raid adapter.
Still it is up and running now.
Thanks for all of your help.
Trevor