Worcester Bosch Highflow 400 Gas Boiler?

Hi
Wonder if anyone can help. My WB HF400 boiler occassionally starts 'clicking' repeatedly. When I look at the circuit board behind the programming panel, there is a sort of gas filled diode or something labelled SG1 and it keeps flashing and at the same time, the relay labelled K2 clicks on and off. It doesn't seem to heat when in this mode. Then, when it settles down, it seems to work normally for a while before it starts again.
Just wanted to check if it something simple before I call WB, as last time I called WB it cost me £180 for the engineer to cut an inch off the end of the plastic pressure sensor pipe, which had a pin-hole in it - could it be a similar probem, I wonder?
Thanks