thank you for troubling to reply - I am most grateful.
I no longer have a BIOS because (with no replacement BIOS on a USB stick or otherwise accessible) I rebooted after having removed the jumper - so putting it in recovery mode.(I vaguely recollect the BIOS (I inadvertently in my laziness wiped in this way) was from 2015.) (I built the machine - with a lower powered processor - around 2010)
What BIOS do I need?I'm afraid I don't know . Whatever will get it working again
Processor Model number: Intel Core 2 Quad Q9400
Why was I monkeying with the jumper?
20 or so seconds after switching on at the PSU (reducing to fewer seconds with successive tries) (with lights appearing on the mobo and with processor fan working) the PSU would switch off. (This started happening consequent on my having installed a third party video card (after having been using Intel's on board video for several years.) (I installed said card having switched the power off at the PSU but with the cable still plugged into the PSU and had assumed I had corrupted the BIOS by installing the video card with the cable still plugged in - for all the PSU being switched off.)