I have in the past experienced difficulty in starting the engine on two occasions. The first after 50,000 miles and then again at about 100, 000 miles. I changed the preheaters on the first occasion and checked the relay and supplies to the plugs, but to no avail as this did not fix the problem. I found that air was getting into the fuel delivery line, and the system had to be well primed before the engine would start. I suspected the fuel heater was the problem. To prove this theory I bypassed the fuel heater by adding another length of fuel line from the inlet of the fuel filter to the inlet of the pump. The engine subsequencly started without any problems. On the first occasion I changed the fuel heater at a cost of about £50, and that kept it going for another 50,000 miles. On the second occasion I took the fuel heater off and ran a few tests on it. I found the slab casting of the fuel heater had become porous around its edges, allowing air to be drawn into it. I thoroughly cleaned the casting and smeared silicone sealant around the edges including the brass insert. The fuel heater has not given me any further trouble since, and the engine always starts first time..
I thought it worth while passing on the snippet of information.
Piers.