Yes, agree, there was a 'Fiesta' built by Motor Caravan Conversions of Berkhamstead. It was founded in the early 60's by a chap called Bill Rix who had been on holiday to Italy and seen the 600T and came home, set up MCC and produced his first camper on an 850T van in 1965.
An 850T van is this
MCC also built the 'Fargo', 'Amigo' and 'Farina' as well as the 'Fiesta' on the 850T chassis. As far as I am aware Fiesta was only based on 850T. The 850T was replaced by the 900T in about 1977 (different restyled front end)
And the 900T was then replaced by the 900E in 1981 (new dash, front disks off the Panda, square rear lights, etc.)
So if it is 79 and based on 850T it's probably a late registration. The 'Fiesta' may well have carried on to the 900T, but my source book is in Italian and I can translate bits but not all.
Also MCC built the 'Shango' and 'Amigo' on 900T Vans, with the 'Amigo' carried on to the 900E chassis alongside the 'Caravelle'. By this time, with the 900E, Fiat UK were employing MCC as the official appointed converter so Fiat UK published the official brochures on these campers.
I've got some pics, but will need to scan first.
Hope that helps