Did you also disconnect all external devices such as mouse and keyboard ?
Have you tested the monitor on another PC ?
Have you connected the square 4 pin lead from the new PSU to the motherboard ?
Have you got an add-in graphics card you can try ?
Mouse/keyboard: Yep: nothing but the monitor connected.
Tested the monitor: Yes: it's the one I have to swap between the dead one and this slow one.
4-pin lead plugged in: Yes, I have - I just copied the connections used by the old PSU. Perhaps I ought to try the old one again.
Add-in graphics card: I removed it for the other tests but shall try it again and report the result.
Replacing the add-in graphics card had no effect.
The PSU comes with two 12V, 4-pin leads so I changed them over, replaced the memory and - Hey Presto! The BIOS demanded that I select a valid boot device.
I shall persevere and report back. Thank you for your advice so far.