Potterton puma boiler hw but no ch?

Hi my boiler will run for hot water but not for ch.
Have fitted new micro switch,and the flow pin connects and disconnects as it should. Turn off hot tap the lcd light goes out , with the boiler switched to ch the boiler will then run for about 30 seconds and then dies.

Then the other night i decided to have a look at div valve, empty boiler turn off water disconect water pipes and remove grub screws.

But could not budge the houseing,then decided maybe not good idea, as looks like i would need washers or o rings when put back together, gone 6pm so plumb shop shut.

So put all back together refill boiler and then the thing fires up.

Not only that but ran for a good hour UNTILL i gave it the big test and turned on hot water tap now am back at square one .

Any help please.