Hi,
Is anything else odd happening? Are the revs staying high in any gears? Have they checked the functionality of the torque converter?
The autobox on yours (N reg ISTR) is not hooked up to the ECU in any way, so it can't be a computer controlled problem, therefore you are looking purely at mechanical failures. The throttle linkages, the torque convertor and final drive.
The gearing is not an issue, and if your engine was bady tuned it would be sluggish, not burning out boxes! I assume when towing you drive in D? (I have heard that 3 is quite useful in the TDi auto with a big van!)
I'll have a hunt about and see if I have anything on file here....