525TDs revs great, but wont go faster than 30!

[High Compression II]

Got a chance of a great 525TDs, but it has weird problem While the engine sound fine, no noises smoke or anything like that, No dashboard warning lights etc. the vehicle will not go faster than 30 miles an hour! which is 1500RPM in drive. The fault I have been told is not intermittent and present all the time/temperature/conditions etc.

Its an Auto with I believe a M51 Diesel engine and 5HP18 ZF Trans, and was registered in 1993.

I need to find if this is an engine based fault--which for me would be OK!, or are the gearbox electronics 'telling' the engine to stay at 1500 as a limp-home mode?
Another issue is the vehicle is around 300 odd miles from me, so proper diagnosis by myself is difficult!

Is there any way of 'disconnecting' the electronic control of the gearbox if this is the fault, and if so, would the vehicle drive, all be it not correctly?

Any guesses/wild stabs in the dark?

Any info or past-cases/on similar vehicles would be interesting to hear!

Im a fair Diesel engineer, and am an electronics technician by proffession.

I plan to buy this as my first BMW, having had Mercedes for many years, and convert it to use vegetable oil as fuel like the Mercs before it.........But thats another story!