Solved it and we have moved.

Continuing the saga, I'd been thinking about the oil pressure sender all night (not really), and I thought if I switch on the ignition and touch the sender wire to earth, the gauge should move all the way to the far end. Low and behold, it did, so that checks out the circuitry as being sound.
Next was a trip in the car to ABC Leisure in Lowesmoor Basin in Worcester, but unfortunately they were out of stock, as they were in Alvechurch.
Right then, after an enquiring phone call we made a quick sprint down the M5 to Beta Marine in Gloucester, and after lightening the load in my wallet I came away with an brand spanking new oil pressure sender.
Then we made a quick sprint back up the M5 during which we noticed the South bound carriageway was blocked due to a broken down lorry being manoeuvred onto a tow truck. That was lucky, we missed it.
Got back to TQ and within a few minutes I had the new sender fitted and working.
SPLICE THE MAINBRACE, WE'RE LEAVING!

In close-up.

And panned out to get a better idea of where it is.

...and now in Netherwich Basin, Droitwich.

Oh! I nearly forgot, Lucy...

Hello... I struggled to wake her. Well she is 18+, bless.