I have a spare 3.89 diff to work on. is it possible to fit 4.1 crown wheel & pinion to this and how difficult, thanks ernie

Yes, dead easy. Hard bit is the collapsible washer on the front oil seal when you put it back together. I always get these professionally done, but I'm sure it is feasible to do it yourself somehow!

Thanks Dicki, is there an instruction sheet on how to do it anywhere about and also the cheapest place to buy th CWP

The Haynes manual tells you how. As for finding a 4.1 CWP they are like the proverbial rocking horse excrement. I have found 2 now, and both were nadged. You can buy them from Burtons new for about £300 incl vat. Try Fast Ford or Classic Ford mags. Occasionally ebay, but no guarantee of quality with any of them!