Right.....
having given it some time to "settle" after the many patient and gratefully received ministrations by the saintly Admin crew.... And of course James's heroic battle with the stat housing, ....

it's still losing coolant..... about a litre every 2 days.
BUT.
there is NO sign of any leaks... even at temperature, after a slow journey in rush hour... with the engine still running... look underneath and it's bone dry... not a drip in sight.
No oil in the coolant either..... or not so as you'd notice, but i'm having to refill the header tank about every other day... (alternating now between antifreeze and water... but it's expensive.... )
NO clouds of steam out of the exhaust either....
it's a bloody mystery to me...... and I've learnt my lesson about handing it over to garages & trusting them to find it quickly.....
the only thing that is somewhat concerning, and possibility related, things are.
1) that we get assorted "hot " water/coolant/oily, smells using the heater (especially on full blast.... ) but no dampness in the footwells... and that in heavy traffic using the heater is required to keep it down to "normal" mid 90's operating temperature.... after a while.... (it now takes a lot longer to reach that point though, having had it's several versions of niagra falls fixed )
2) that stuck fan/pump/whatever squealing pig noise happens once the engine is switched off, if the car's hot.... you know when a cooling fan would normally come on after you switch off, ... but instead we get this horrendous noise...... which i can stop by turning the ignition on 9but not starting the engine) so that the internal heater cools the systems somewhat...
On a "run" it behaves impeccably.... temperature steady , any rises seem to be dealt with by primary cooling fan as and when required....... it's when it's been running for a while, then has to sit stationary in traffic that it begins to get toasty....
any ideas?

Max