I am not too sure about the boiler spark triggering the sensor, the
boiler is quite old and has a permanent pilot rather than a spark
thingy (sorry I don't know what this device is called but know what
you mean...I think). I am not the market for a new boiler, if I was
then it would be the condensing type, then again slightly off-topic...
what's the average to get a return on a condensing boiler?

I have also considered moving the sensor lower than the flue outlet,
but this would be too low and IMO an easy target for anyone to get at
it (less than 2m). This unfortunately seems to be my only option,

Many thanks
Naz