IDEAL CLASSIC BOILER ONLY STAYS ON FOR 2 MINUTES?

My Ideal Classic FF280 will only stay on for 2 minutes then cuts out.

I have recently been having problems with radiators, which I suspected was sludge, so I switched everything off, drained down the system (Open vented indirect, cylinder on 2 floor, 2 tanks above in attic, boiler in bedroom close by), added the cleanser and topped the system back up. I bled everything, no problems there, I bled the Central Heating pump (Grundfos Selectric 988123), no problems, the boiler (air release valve on top of central heating pipe, no problems and a bleed valve next to the cylinder.

When I started up again, I switched the central heating on but left the boiler off, let it run for an hour, then checked all bleed points again. Once I was sure I had got all the air out I switched the boiler on. I got it to produce a tank of hot water, but the radiators didnt heat up. The motorised valves definitely work, and the central heating pump is working.

So now I have a situation where the boiler is only igniting for 2 minutes. What I have done now is switched the immersion heater on to heat the water, then I was going to switch the central heating back on to see if the pump will push the water to the radiators to prove that there isn't a blockage anywhere.

Thats where I am at now. Any advice appreciated on methods I can use to identify where the problem is at.

Many thanks