Advanced transmissions in Billingham (as recommended on here ) offered these thoughts...
The huge clunk will invariably be the front drum cracked inside the box. Apparently its a classic tell tale when in limp mode.
He doesn't see a lot of gains topping up fluids etc as , having done "20 -30 of these boxes when most people wont touch them" that is where the finger of fate is pointing.
Very helpful chap. Did also give me a heads up that the manifiold bolts can also prove problematic as part of this whole repair.
Our site sponsors will pay £500 for the car as it stands as a breaker.
I had just advertised it for sale for £3200 (its Ollies old car if that makes any difference to people knowing how meticulously it had been cared for - including gearbox oil changes...)
£2942 + vat for reconditioned box = £3530.50
Ho hum ....