I own a W plate Bongo with gear box issues

A highly recomended local auto gearbox speicalist has been looking at it. His tester said that there was a fault on solienoid 2, which was duely ordered and replaced, but has not cured the problem. Upon further investigation swapping the other solienoids and testing has shown no difference so he called his electrical specialist in to check the wireing etc. He has found no faults either and they now beleive the fault to lie with the "computer" which controls the gearbox and body. I have no experience in this area at all but I'm told that this was formally called the ECU and is now the TCM or transmissioon control unit.
However, trying to locate this unit for a year 2000 Bongo is proving very difficult. If anyone has any advice, or has had a similar problem they've solved I'd be grateful to hear from you as my beloved Bongo has now been off the road for opver 3 weeks! The gearbox has already been flushed and new oil etc installed but ths hasn't madeany difference.
your hopefully,
Simon.