Worcester heatslave 20/25?

Hi, I have been having a problem with my heating for some months now but very intermittent, occasionally the heating/radiators come on for no reason when the heating is turned off on the boiler. I called the engineers who service the boiler every year but unfortunately as it was OK when they turned up, they couldn't check it and said to call them next time it did it. Unfortunately it has done it since but always outside of office hours... 3 'o'clock in the morning on one occasion. Now the three radiators in the lower part of the house (the lounge) are not coming on. Upstairs are coming on. The inlet pipe leading to two of the three that are coming on is hot but the third and I suspect the last radiator on the system is cold. I have bled the radiators but no problems, no air, no noise. Could it be a dodgy diverter valve, would that cause some radiators to come on and others not? Incidentally I have checked the thermostats on the cold radiators, they are all OK . Please help.