That's what I get for speaking too soon.
After performing a reboot while installing XP SP3. HDD light solid on and doesn't even POST.
Don't really have the time to look at it right now but I'll have another stab at it tomorrow.
Tried all the usual stuff e.g. disconnecting HDD/CD, checking to make sure DIMMS are fully home, power connectors from PSU to mobo.
First thing I'll try tomorrow removing HS and check to make sure the AS5 hasn't come into contact with anything other that HS/CPU.
Any suggegstions welcome but in my experience no POST = can't see RAM or can't see CPU.