Ideal Classic FF350 doesnt heat to required temp?

Hi
We have an Old cottage that is used as my office, now its getting cold ive noticed that the rads dont heat up much above 50c and the boiler only runs for a couple of mins before stopping and cools down a bit and starts again. Its not the over temp thats cutting it out seems to think the water is upto temp but the flow pipe is only anout 50c as well.
Ive seen this asked many times but no one seems to post back that they fixed it..