I had turned off the boiler when we went on holiday but it will now not fire up. There is a bubblng sound as it tries to fire up but eventually it goes to lock out, i was advised that there is no water getting to it and i must have a blockage, so i have had the system completeley flushed and all the rads off and flushed out, the boiler did fire up for about 20 minutes but it was still making the bubbling sound but the rads did start heating up, but now it is back to firing up then eventually locking out, the plumber who flushed the system didn't think the system was blocked as he drained the system from the return and the flow of the boiler until the water was running clear has anybody any ideas?