thanks for the warm welcome all!
1940s/50s! that's older than the blooming car - no wonder it was seized open
anyway i flushed the system in both directions (including the heater) until nice clear fluid came out and filled up with 33% bluecol. just need to get some revs up to clear the airlock in the heater (i hope!).
radiator is now behaving normally now, flushing suddenly with very hot water so chuffed with the results - but just need to get the heater bloomin warmer (the whole reason i was tackling this in the first place).