I managed to get a little further yesterday. I cut out the helmet arrows and then painted fiberglass resin over the exposed card around the rest of the helmet. The ides was for the resin to soak into the card, and allow me to sand it smooth to a nice finish without furring up the card.
After my last (failed) attempt at the same technique on the earpeices, I used extra hardner this time.
I'm happy to say it worked, although much sanding was required. The edges are much sharper.
I still need to so some more filling and sanding on the rangefinder and stalk, and I'll probably coat them with some resin too once they're good enough, but it's getting there.
Edit; As you can see, I didn't coat resin on the area to be cut out for the T-Visor... I thought there was little point, and may make the cutting out more difficult. Speaking of T-Visors; I also spent a couple of hours on google, trying to find a UK based supplier for the shade 5 Green faceshield; they all seem to be the wrong size; at 8" horizontal, with varying vertical sizes. I guess I'll have to keep looking.