East Kent sell a pack of U clips for the seal. However, they had a selection of different ones on their stand at the NEC, all apparently for a Viva / Firenza in November.
I bought these ones as they seemed to be the deepest and would probably fit the best. I haven't fitted them yet so can't comment on their effectiveness. I fitted the seals on my brother's Sports Hatch using a different one from my stock of 'unidentified parts that have been collected over the years' and they worked very well.
At the minute mine are fitted with 4.8 mm stainless steel 10mm self tappers which are caught through the existing slots in the door and trim. It works well but needs checked for tightness every season. Drilling the door for a new fixing would be the best idea but risks of damaging paint with the drill chuck are quite high.
In fact it works well enough that I may not bother to use the U clips I've bought!