Thanks very much for your comments.
Londonboy - I did consider separate controls in each flat on installation but rejected it due to complexity - particularly as we couldnt use conventional 240 volt controls as each flat has its own mains supply and you shouldn't mix different mains supplies...
You are right Agile - design temperatures for living areas should be more like 22 than 20.
I checked the design and actually the design temperature was 17 deg C in the stairwell and 22 deg C in the flats against an external temperature of -2 deg C. I have no complaints from tenants so am...
I have a large house divided into 9 individual rented small flats with a communal central heating system system. The boiler is a Worcester 30SE which works fine - the radiator sizing and pipe sizing seems to have worked out fine and the rooms stay reasonably evenly warm even when very cold...