As you say, the Longman/Curnow Datapost cars are probably the best known BSCC/BTCC Fiestas, but I think there were quite a few more- you've mentioned David da Costa's car (is this the pic you were talking about?)...
http://www.flickr.com/photos/1678887...57624382877618
....and Guenda Eadie (who came from the Faberge Ladies Cup one-make Fiesta series- the same competition that discovered Louise Aitken-Walker) had one in 1980.
The other one that comes to mind is the red & gold Julian May/Pegasus Mouldings car, which appeared between about 1980-83, with a few different drivers, including Terry Nightingale and Jock Robertson.
http://img171.imageshack.us/img171/1451/scan0016lm0.jpg
http://i83.photobucket.com/albums/j285/Co-L-d/lg_1239_1acd.jpg
As I remember, it raced in both 1.3 and 1.6 litre form and is still around- the current owner posted some pics of it on here a few months back
http://tentenths.com/forum/showpost....5&postcount=98
Think the last Fiesta seen in the BTCC was Alistair Davidson's 'Troy Ambisonics' car which did a few rounds in '86- IIRC it was actually a Fiesta Challenge car, and would have been a Mk2 by then? Don't think I've ever seen a pic of that one!