I registered for the race under ‘C’ (also corresponds to my category) -
but was classified in group ‘B’ on Zwiftpower.
Can anyone tell me where I can appeal against this?
Best regards
If you look at the results page on ZP you will notice within the race information this line:
“A B C D E (Riders exceeding w/kg will be upgraded)”
Most likely you entered a race governed by non CE regulations and as such the race organiser has introduced this rule to move/promote anybody racing above the category w/kg limit.
I doubt you have any appeal opportunity.
Thank you, Ian!
that makes sense to me.
… and always read the small print! 
Keep On Riding!