Thanks for the hints... I am becoming baffled with this ...
The scuttle drain was blocked - now dripping happily on nearside wheel

Bonnet now opens and is adjusted. Haven't solved original problem.
There's now a slow but steady drip drip of coolant from approx front of gearbox, driver side (near the downpipe on the drivers side)
everything in the engine bay appears fine - and very *DRY*... except:
The bottom hose from the bulkhead runs to a T piece - the top of the T is kinked over and then runs to the bottom of two connections on the right side of the HBV. It's certainly enough to restrict flow heavily - maybe enough to nearly stop it. Dunno if that's the cause, or caused a pipe to pop off inside cabin due to pressure.
All of this & HBV are bone dry. This is 80% by touch - access is not the best.

The two connectors that run forward from the HBV are the two I need to link, yes?
Think it's put off til morning anyway as I have no torx sockets to take plenum off - didn't bother to look before going to halfords for hose and torx bits (separate trips of course!!!)

Oh well I learnt plenty. Hols postponed til tomorrow