Spraggo is spot on, god only knows why the plumber you got out couldn't suss that in about 5 seconds flat, don't use him again! If you remove the large chrome slotted screw/vent plug from the rear of the pump (there will be a trickle of water, get a bowl or some rags) and put a straight screwdriver in the hole, it will rattle when it touches the shaft of the pump, if it is rotating. If the pump is not rotating (which is my guess) then try to find the slot in the end of the shaft and turn the shaft with a screwdriver. The slot is sometimes not very well formed, but it is possible, use a torch and try to find a screwdriver that fits well and is the same length as the slot. push the shaft in and let it spring back out a couple of times, and then get it turning freely. Switch the pump back on, and check it revolves (turn the speed to max for an hour or so) and all should be well. Refitt the screwed plug,The other possibility is that the boiler or the main circulators are sludged up, and this sludge is preventing circulation and thus the boiler is cutting out on the boilet thermostat. How long since installed/flushed? if its an old system this is a possibility. Is there a drain off at the bottom of the boiler, or on the return near the boiler, if so you could try draining some water out of there to see if there is sludge.......BUT BEWARE!!! the washer on the drain off tap will probably break up and leave you having to drain right down to replace it, so dont try this unless absolutely neccasary AND you have a spare washer!
Thanks so much for all the help guys...really appreciated would definitely try your remedy, but simply cannot get to the back of the damn pump...it has been built (along with the boiler) into a rather tight spot in a brick shed and is pretty tight up against a wall so I can't access the back of it...maybe this is why the plumber said he didn't know what was wrong...because it was going to take an age to access the rear of the pump? Plus....if on the odd chance I develop triple jointed limbs and manage to get to the back of the pump...should i turn off the central heating system at the wall swtich in the house or do i need to do something more drastic with the electrics & water supply & oil? (just checking to make sure I survive till new year