Defender Common rail problem, help!!!

Hi, my 2007 defender has a problem that started June 4th this year, going to N/wales to do a job, steaming along at 70mph and suddenly it starts surging and speed drops to 50mph but then I was able to continue as long as i didn't try and go faster. Since then it starts normally goes like a train for the first 10 - 15 miles as fast as I want then drops down to 50, no smoke, temperature normal. It has done many miles with this fault and consumed over 350ltrs of fuel, it has also been serviced and the dealer couldn't find the fault. Then i took it to a closer LR dealer and they say it has been filled with petrol at some time (it hasn't) and needs a new pump and injectors at £2,500. I say I'm well aware of the damage petrol can do to pumps and injectors but even if that was the case it wouldn't start and go like a train for 15 miles then play up it doesn't fit the symptoms, can anyone offer help as i get the feeling the dealer is just going to change everything without knowing why or which part has failed, high pressure regulation valve maybe?

best regards
George

Sounds like the ECU is going in to "limp" mode. Stopping and restarting the engine resets the ECU but there should be an error logged which the dealer should be able to read on their diagnostics.
Common causes are turbo overboost or engine overheat.

HI, no it doesn't go into limp, no faults in the fault log, goes like a train at all times but when fault develops it will not go above 55-55 on the motorway, it will go all day at low speeds and accelerate at full power off motorway but hitting 50 is like a barrier it won't go through. as I said, starts OK, no smoke, temp is fine, its had about 100lts of diesel through it so its not bad fuel, been serviced too but they said there was no fault with it(only 'cos they didn't road test it for over 10 miles or so) now second dealer is trying to say it needs new injectors and pump at £2,500 out of warranty, trying to say I wrong fuelled it, !!!!!!!