Have you balanced the radiators yet?
When you say balance do you mean open and close inlets and outslets to gain optimum flow - is there a set procedure for this as i can see this being a pain.
Have a look in the "for reference" section or do a search - there are good instructions on here. if your pump is working properly it ought to be able to pump hot water to any of the rads. Also, make sure that you can prevent hot water being supplied to the hot water cylinder once it is hot enough (look for the thermostat on the side, turn it down, feel the supply pipes to the coil - if hot water keeps going to the cylinder there may not be enough flow remaining for the rads. Start off by turning off all the hot rads and see if the cold ones go hot, or if they are still at the bottom or at the top.
Is there air in the system? well, is there? When you bleed a rad, what colour is the water? yes the water is normal coloured
If it not black or brown, this is encouraging - corrosion usually makes black material.
Do you have thermosataic radiator valves? If yes, in which rooms?
on 4 rads out of 7 rads
2 up 2 down
one of the prob rads has one.
Do you suppose the prob rad might have a sticking valve? Take the head off and see if the pin goes up and down, But do the balancing first.
Where is the room thermostat?
on the stairs we never use it its set on max
Is that because your CH sysytem has insufficient power, or because you like throwing money away? 
Is there a Feed & Expansion (header tank) in the loft or airing cupboard?
yes in the loft
then you don't need to use those little tubes of chemical, you can use the 1 litre bottles and pour them in the top of the tank
And, about how old is your boiler and the rest of the system
as old as the house Mid 80's
Probably full of horrid gunge then. Try draining and flushing it to get some loose stuff out, then a cleaning chemical and drain and flush some more; then when you think its clean, refill using inhibitor. Have you found some drain-off points?
Many thanks for any help
