I've had a slow coolant loss over the last month, at the rate of ~1 liter/1000Km. No signs on rad, hoses, oil cap, or engine, so it started to look like the head gasket .
I did a cold pressure test - and it held 1 bar for 5 hrs: repeated today on a fully warmed engine, and finally the pressure started to fall - and a small pool of coolant formed under the firewall.
It appears that the expansion tank is the leaker, dribbling down the center mounting foot. It is not leaking from the overflow, cap, level sensor, or hose connection: must be a failed seam in the tank. Much preferable to doing a head gasket, but seems to be an unusual failure ?? ...So a new tank is in order - any recommendations as to the better place to find one ( also a 1.4 bar cap to replace the 2.0 bar one the dealer sent me previously).