How can I stop my Ravenheat CSI 85B tripping out?

I had a new Ravenheat CSI 85 B T installed today. It worked for the plumbers who installed it - they left and went on to the next job.

My previous boiler (820/20) had finally given up the ghost after 15 years and this model was going cheap at Plumbase in Loughborough

So, the day went well. The plumbers left and I thought I'd give the system a check over.

My previous boiler was incapable of runnig a hot bath so I thought that would be a good test.

Half way though filling the bath the boiler tripped with the pilot/overheat light flashing. This fault quickly became worse and now when either DHW or CH are selected. The fan starts, the pump starts then after about 10 seconds there is a high pitched note and the boiler trips. As far as I can see the boiler never fires

I dont know how to tell if there is a spark but I'm in the area on Ravenheat's flow chart of the condensing sensor, air flow pressure switch and gas valve.

Ravenheat have got me pencilled in for a warranty visit on Friday - in the mean time it's cold showers and a wood fire for comfort!!

Can any one please help me to further isolate this problem so I can at least ensure that the Ravenheat man brings with him the most likely causes of this fault