My automatic gearbox will not pull away in D but will manually in 1st and you can gather momentum and shift all the way up to Drive eventually. I self diagnosed this as a blown gearbox? Am I correct? Problem is I've now purchased a donar vehical which drives fine until you put it into low ratio and then it vibrates and just doesn't sound good when you gather any speed / revs whatsoever. Could this be the transfer box? Between the two faulty vehicals could I make one good? Any help would be appreciated.

Failed internal clutch, see https://www.ashcroft-transmissions.co.uk/frequently-asked-questions/4hp22/4e-faq-s.html. On your other one, if it is OK in High range but only shows a problem in Low, that is a transfer box problem. So yes, you can make on good one out of two. Make sure you change the gearbox output shaft seal when you swap the transfer boxes over. RAVE says you should always do it if the transfer box is removed, I didn't and having to take the transfer box off again just to change the seal is a real pain.

Many thanks for your response, that's good news.

Sprag Clutch A Failure i'm afraid..

Quite common on the HP22 not so on the HP24 but if abused it is possible.