For those who may be interested, I appear to have found the fault with my FBH.
I had a bit of time to play about with it this morning and it looks like I may have an intermittent fault with the water pump attached to the side of it.
When testing it this morning, it fired up and ran for approximately 5 mins before going totally silent. This didn't seem to tally with any of the timings that French Mike has posted on the internet. I had a good look around it, proding and poking when I noticed that the water outlet pipe was extremely hot. I started the engine and it immediately cooled down. I then turned off the engine and fired up the FBH again, the outlet pipe was getting hot again so I tapped the water pump and heard a bit of gurgling. I started the engine to help with the water flow and the FBH continued to run until the engine was up to 72 degs (maybe 15 mins). I switched off the engine and the heater carried on running till the temp was 74 then shut itself down.
So, I feel marginally better now as I know the problem is nothing too serious. I can remove the pump and strip it down, maybe even fit a new one.
My GSM unit arrived yesterday so I can now make a start on fitting that too.
So, if I can offer any advice to anyone, check water is actually flowing through your heater if it shuts down too early. Without flowing coolant, the heater will think it's done it's job by getting the water up to 74 Deg. It has in a sense, but only a very small amount.
Thanks for listening to my ramblings.
