From a quick scan of the data sheet I think that's the same material I used Joe, but with a different name/supplier which isn't surprising I suppose as we're on opposite sides of the pond !
I'm not sure what's meant by "polyurethane" primer, I'm not in the painting game and don't keep up with things. I have seen the term associated with water based paints so I think I'd ask around and see if the usual GRP painting specialists use it. It could well be the modern generation primer and ideal for the job, it's just new to me.
The generic term for what I used is polyester spray filler, the same material as Buzzer used from his post. It's basically resin with filler and sets by chemical reaction rather than solvent evapouration, which is what a lot of older "high build" primers used to do and they were the ones that used to sink after a few months. The brand I used was U-Pol Reface which has been around in the UK for a long time now.
Brian