Ok, think I'm sorted.
Dropped the coolant (distilled water) & it was still a bit dirty so I stuck a hose in the expansion tank & left it running for 5 - 10 mins. Reconnected the bottom hose & put in 5 litres of mixed coolant. Since there was bound to be some water left in the system (probably the heater matrix I put in a litre of neat coolant then another couple of litres of mixed.
I fired up the engine with the expansion tank cap on & the bleed tap open. At least I think it was open - I've had it this way & that so many times I've lost track which way it was originally. I let it run for 20 mins (with the heater on 'Hi') then let it cool.
The tank level dropped to the min position so topped up with mixed coolant & started the engine again. By now the heater was putting out the kind of heat you would expect from a vehicle that could be operating in a cold US state in winter.
I still don't know if opening the bleed tap made any difference. The coolant system in my R129 SL is self-bleeding without the need for a bleed tap so why the WG has to have one gawd knows. I think the heat/cool cycle may be enough (and patience).