Many thanks to Ian and Rob for their considered advice.
However, I should like to put into perspective the approach to carrying out our own diagnosis of the fault.
The isolation of the pressure switch by removing the plastic tube from the fan housing whilst the boiler was running, caused as...
My son's elderly Potterton Kingfisher has suddenly developed this problem whereby the main burner flame goes out and doesnt re-ignite. this started happening during the recent gales and we therefore put it down to turbulence arround the balanced flue terminal.
Since the winds have subsided...