Hello Siatbury,
at Stridaforum!
Anyway you have to unmount the little roller (snubber bearing) at the rear.
Maybe this post helps a bit, principially there’s not that much difference to your Strida 3:
Re: fitting a new belt on strida 5
The adequate reply for the white van man would be: “Sure mate - and I’m kickboxing as well!” 
Kind regards,
Chris