Worcester 24i - Cold Radiators!!!?

Hello
I am having problems with my central heating system. Basically the radiators are cold even when the heating is switched on. I have a worcester combi boiler.

When I initially switch the heating on, the boiler lights and the heating flow pipe turns hot. However, after 90 seconds, the boiler switches off for 4 minutes. This repeats constantly meaning the boiler is never on for more than 90 seconds! I have lifted the floorboards to locate the manifold as I was told there could be a problem with this being blocked. However, it seems as if the 90 seconds of hot water are reaching this and beyond.

I have been informed that the problem might be a blockage in the system which is causing the boiler to think that it has reached the required temperature which is why it is switching off. But the return flow pipe is never getting warm or hot.

Does anyone else think that this sounds like a pipe blockage or does it sound more like a fault with the boiler?

Thanks for any help I can get.

Dave