Cracked it!
Disabled the L2 external cache in the BIOS and it worked.
On the P55 mobo you have a Level 2 external cache. This can be onboard chips or a cache expansion slot - the latter taking a Coast cache module. It looks like the surge has knocked the module out.
I've now reinstalled most of the basic software and I've had no problems but I'm not happy with keeping the mobo in case other components have been weakened. Once I get the HD data verified I'll replace the motherboard.
I've now checked the original memory and all four SIMMS are working fine - as are both original HD's. The only casualty in this event is the L2 cache (and the corrupted data on the disks).
Thanks for everyone who chipped in with suggestions - it did help clear my mind of inessentials..