Thanks, glad to know the backfire was not likely a product of some sinister fault.
We do need to check for spark properly, so far we have just assumed that it is not sparking since it is getting fuel, so that would be the likely suspect, will have another look at it and hopefully make some headway with it later.
Most of our earlier time last night was trying to find and fix a water leak so we ran out of time to look further into the starting problem.