That's disappointing about Solihull OPC. I use them a fair bit for parts and they always call when promised. Must be service dept that's lacking!

Interesting on the new leak. It's often case that when a failed cap is replaced, it will reveal leak(s) elsewhere as the system is now being pressurised. Realise it must be frustrating for you as I'm sure you wanted to be able too confirm whether or not the cap was main cause of coolant loss.

Leak from rear of engine is most likely the short hose between block and heater valve kenthepipe mentioned earlier. Also check whether any leak from bolts (2 per side) at rear of each head. Left side is a blanking plate, right side is that heater valve hose connection. They are accessible with air filter box removed (take off cover, air filter, then the 10mm nuts on base, disconnect hose right side).

The air filter is replaced every 30,000 Miles on official schedule, so may not have been part of otherwise comprehensive service.

So are you saying you still see drips immediately after starting from rear off engine bay?

With the undertrays on (at least you have them!) the drips you see could be from original source some distance away. Any chance you could get your car safely on stands at front and whip them off? There is no source of coolant at rear other than those I described. The only direct source low in engine bay is lower rad hose where meets rad, the water pump, and of course base of radiator. Is it original radiator? They often need replacing around 20 years.

Do you have a record of when water pump was last changed?

With a leak immediate on start up (so will leak whether your expansion tank cap is good or not), suspicion turns to pump. May not be leak in pump itself but through one of the bolts, or the seal to block.

The pump sits behind the centre timing belt cover.