Hi, I've now had a chance to try out both PSU and Graphics card in a second system:
Results are:
When using my PSU and graphics card, the system failed to reach BIOS - no beeps, blank screen.
We then swapped to use my friends PSU,the system failed to reach BIOS - no beeps, blank screen.
We then swapped my graphics card for his - the system booted immediately.
That seems fairly conclusive that the graphics card is at fault.