I checked over all the wiring and found a plug/socket not connecting. This is located on the right hand side of the engine top, about 2 inches up from the oil level dipstick. I found the female plug was slightly damaged in that one of the short sides of the rectangle was broken, allowing one leg of the security spring to jump inwards. This prevented engagement of the plug to socket. I managed to prise the spring out (it's a strong bugger) and get it into its correct location before joining the electrical connection with a resounding 'click'. The engine then started! It was a bit lumpy at first, possibly due to the excess petrol vapour that had accumulated during the multiple starting failures, and which could launch the space shuttle. All is now good....except for the heavy oil leak that I thought I'd fixed! Many thanks to all of you who offered advice. I have learned a lot and I hope others will learn from my experience.