my gas central heating radiators are not warming up?

I have a gas back boiler system(indirect system, I think). It was running OK even during the summer and early autumn.

recently the heating was giving trouble...heating upstairs rads and not downstairs..and then, all rads OK..then..it was just top rads..or if they are turned off..the bottom rads will be ok..but, if that are running and I turn down the room thermo, it dies down..its a matter of lucky dip...

The heating of hot water is OK...just the central heating thats not working properly..

Sometimes I think its the sliding switch that is causing the problems..as the pump seems to be running..the pipes above and beneath the pump are hot..and sometimes, I think maybe the pump may be slowing down..

I have bled the rads..but, the system had not been service for 2 1/2 yrs...although no problems had been encountered until now..just before xmas..

Could it be that the back boiler is dirty and cleaning/servicing? could that contribute to the problems?

any help please?

mike