Help, please you experts!  My Ideal Combi Mini  HE C28 is only just over 2 years old.  A few days ago, shower suddenly went cold for a few minutes, then got hot again.
I proved it was not a shower problem by running a hot tap in the kitchen and this ran hot for a minute or two, then went cold, then hot again.  The boiler has warning LEDs which flashed a code for Air Pressure switch failure.  The boiler cuts out after 1-2 minutes.  15 seconds later the LEDs signal the problem and about a minute later, the boiler re-ignites and the cycle repeats itself.
I called a local heating engineer.  He started by checking the cleanliness of the fan venturi, which was OK.  He then disappeared off and retuned with a replacement air pressure switch which made no difference at all to the problem.
He did all the other stuff, which I guess is normal such as adjusting the  gas pressure etc.  He remarked on how clean the interior of the boiler was.
Having spent a couple of hours, much of it reading my Ideal installation and servicing manual, promised to return the next day.
On returning the next day, he measured the vacuum created by the venturi which measured 1.4 mbars on boiler ignition and over the next minute or so dropped steadily to 0.7 mbars at which point the boiler stopped again.  
With the inner case cover removed, the problem disappeared.  
My engineer clearly is stumped by the problem, and has now spent close to 5 hours at my home, and keeps muttering about the eventual bill.
His last suggestion is to replace the fan (£150.00??) to eliminate it from the problem but doesn't really inspire me with any confidence that this will cure it.
I have 2 problems 1. The boiler is not fixed and 2. This guy is going to present me with a huge bill for no resolution.
The engineer seems a really nice guy and I have no reason to believe he is deliberately ripping me off.  But should I be paying while he stumbles about trying to fix it?
Help, help, please  

'With the inner case cover removed, the problem disappeared.'

This would suggest to me that the fan is struggling to draw air in from the outside which it does by creating a vacuum in the boiler when venting.