In case anyone else is ever after one, I found them on Ebay ....

here -> http://cgi.ebay.co.uk/FORD-SIERRA-in...3A1%7C294%3A50

The correct size is 40/75/12 for the pinion, many bearing places keep 40/75/10 in stock but not the extra 2mm deep ones.
I'm hoping it isn't the washer missing off the pinion flange ( the bit you have to pull off to get at the seal ). Anyone got a pic of the flange with the retaining nut removed?