Thanks, Al, Mike & John,
John, the rad does not have its own header tank or cap. Its like a long
honeycomb tucked well behind the front grill, about 2 feet wide and
about 9 inches deep. and both inlet and outlet hoses are at one end.
Unfortunately the end which is totally covered by the air filter and
other assorted pipework EGR pipes ect,running down to the turbo
intercooler.
No access at all from the top of the engine bay , cannot even get one
finger to the bottom hose from above.
The rad is not down at the bottom of the front grill space, its
relatively high up With the turbo intercooler radiator below it.
In days gone by, more simple cars, I used to pull the bottom hose when
the rad was lower, with a hot engine, thermostat open and most of the
coolant came right out.

Al, the 1.5 litres is part of the 6. Interested that you consider the
remaining old fluid not critical. I had hoped someone would comment on
this.
Its all ( well mostly) fresh coolant right now so I wondered if a
regular partial change plan running from now would be ok.
On the plus side I thought the cooling system would be getting a
regular infusion of fresh ethylene glycol. I dont know what happens to
the dwindling molecules of the old coolant and additive though. Like,
does it turn into acid or something?
Mike, Good idea re the tester, I will get one.
Maybe I am spending for too much time thinking about antifreeze but
it was a grueling experience doing our 2 cars the other day.
Thanks everyone for commenting on this,