Baxi Bahama 100 - Hot water problem?

Hi there to anyone who can possibly help me with the above problem.

The boiler works fine for central heating, the problem is with hot water. The boiler is very hit and miss lately with firing up when a tap is opened. Sometimes it works fine at other times
I find that a gentle knock with a spanner on the mains cold water inlet pipe gets it to fire up when a hot tap is open.

We have been away on holiday and this problem has developed since we have been back, over the last few days. The heating was left to run as normal program, but no taps were in use.

I wonder if some crud has managed to block the operation of the flow detector on the mains inlet? Is this feasable?

If so, can the detector be removed and cleaned and reinstated? Is it a screw thread to remove it?

I have over the time we have had the boiler replaced two ignition PCBs and one main PCB, when the auto-vent let water pass and it ran down the cables onto the PCB connections. What a good design.
Needless to say the auto-vent has been replaced each time.

If the above assumption is incorrect, any other suggestions as to the cause/fix would be appreciated.

Thanks in advance for any possible solution/knowledge.

Tonynat
May 2008