This could be the GPU sensor but I'm kind of thinking it maybe the NORTHBRIDGE chipset sensor instead.
 
Do you perchance have a passive or active cooling solution for your chipset on the motherboard ?!?

Hi, thanks for the reply. I use a standard P4 heatsink and fan for the CPU with an additional case fan. The northbridge has a smaller heatsink without a fan - you can see the mb layout here.
 
Would the bios shut down if the northbridge was overheating?

Since you have passive cooling on the northbridge I would think that the only way to verify this is to try a different graphics card and see if that same sensor reads the same temp or not.
 
This will then tell us if it related to the graphics card or not.
 
Another thing to note is that most passive coolers have thermal tape instead of a nice thin layer of thermal transfer compound. It's possible that this tape is not working so good and hence the reason that sensor is reading high.
 
Could also mean the the sensor is not relaying the correct information back too, hard to say until you can determine what the sensor is supposed to be reading

Yeah - last night i took the graphics card out and the temp reading was still there.
 
I tried suspending a fan above the northbridge to see if the temp dropped but it had no effect....
 
Dazed and confused...
 
 

Then either the sensor really isn't there are the sensor is sending incorrect readings back to the app.
 
I would suggest sending this information to ASrock and see if the motherboard sensor/sensors are properly supported by the app.