Took the car back to Warrington Autogas for it's service a couple of months ago.
It turns out that the service is less of a service and more of a "safety inspection" with a gas filter change.
As an amateurish mechanic I could have done this myself, possibly not conduct a thorough and professional gas inspection, but certainly change the gas filter, which I found here http://lpg-kits.org.uk/shop/lpg-filters-service-kits/cat_38.html for £14, so I don't think I'll bother with the services in future.
However, one positive thing that came out of the service was that Andy, flashed my ecu with the latest map. From what I could see happening, it appeared that the new map for the lpg ecu allows it to communicate with the car's ecu more effectively.
Anyway after this new map, there was no stalling, no kangarooing, no loss of power anywhere. The car runs perfectly on gas now, just as if it was on petrol. In fact, sometimes after i've filled up and am still running on petrol, I forget becuase it runs so well on gas now.
In short it's good news, but it should really have run like this from the beginning, given the price. I suppose the main thing is that its fixed now and running perfectly! Now all I need to do is sort out the water leak!
View the whole blog here http://lpgdreams.blogspot.com/