Hi again, with all the radiator bleed valves closed and no leaks, when I opened the tap at the bottom point of the system it was perfectly happy to vent water because of the pressurised system. The system was still pressurised after I had vented 1.5L of water (0.7bar) so, I would guess that...
I have drained out 1.5 L of nice clean fluid from the lowest pont in my central heating system with the idea of inserting 1L of Sintinel X100 into the topmost radiator. Well, all I have now is a 1.5L air bubble (somewhere) and the top radiator is still full of water! The system won't start...