Seems like I got a happy ending. Assembled new mobo using old PSU - which seemed good according to volts reading using my loadtester. Installed Win7 on new 2TB drive.
Connected "broken" 250G PATA drive via the PATA/SATA adapter. Win 7 Partition Editor showed it as an empty drive. So did GPartEd from Parted Magic.
Before trying to restore the MBR I backed up the whole 250G twice (2 x 70 mins approx) using dd on commandline from Parted Magic LiveCD. Identical md5 sums (2 x 25 mins approx) - sounded promising.
When old PC had started misbehaving I had backed up the MBR and Partition table using dd -
dd if=/dev/sda of=MBR.dump bs=512 count=1 as per
http://www.unixmen.com/save-and-restore-mbr-in-linux/So with fingers crossed I rewrote the MBR/Partition table from my saved file - and to my amazement it worked.
I can see all my FAT and reiserfs partitions and lots of files - which I suppose are all present.
From now on I'll always make sure that I've got a backup of the MBR/Partition tables.
And it seems my worries about UEFI were groundless, although maybe because I installed Win7 32bit on (only!) a 2TB disk, and not a bigger one.
The UEFI BIOS interface is all nice and graphical, and Win7 created an old style MBR disk, rather than a GPT disk.
The mobo was a bundle from Currys/PC World outlet on ebay - and am well impressed with what I got for £77.32
ASUS Intel Dualcore G620 Motherboard, Silver Plus Edition Bundle , to which I added a 2nd 2GB stick of RAM.
Cheers,
Peter