Thank you Blackstridaaustria.
Thats exactly it. I’d been searching for alloy crank wheel and alloy crank set, wouldn’t have thought of chain wheel. Very expensive though - 25% of cost of the bike, 
not sure if it’s worth it.
I wonder if it can be bought without the cranks, I really only wanted the chain wheel. Might just buy a new plastic one instead.
Thanks again.
Strida 5 (or rxdavis - depends on what the form lets me log on as). 