So, a bit of an update on this.
The kit is now in Andy / Outrunners hands and it seems I've been the victim of a poor instruction / build order issue. I don't know how I didn't spot it myself but I guess after the early setbacks my heart wasn't in it.
I was trying to assemble the rear arches as instructed, with the flimsy PE trying to carry the weight of them. However, several steps further on, when the bowser tank is added, it seems the arches connect to it, giving all the support needed.
Andy's suggestion is to flip the build order around, mounting the arches onto the tank first. That way the PE could be added later once everything is solidly located, and it would be mostly cosmetic rather than structural.
Again, I'm a silly sausage for not noticing it myself, but well done to Andy for figuring it out, and I'm sure he'll make a fine job of it