Ask the shop owner
to get one out of the packet. Bring a torch as well so you can shine it at it to find the reflective bits and see if you're happy!
I can't imagine there'd be much of a problem seeing it in the dark to be fair. Reflective panels spark up the moment any kind of light hits them (brake lights, headlights, street lights). Unless you live in the middle of the countryside where it's pitch black anyway I don't think you'll have a problem with people not seeing it!
Regarding permits - any chance you could talk to the local council/permit issuer and explain the situation or give them your number plate instead? Seems a bit odd having to get a permit for a motorbike anyway, but I can understand how ridiculous some councils are! If it's still a must then I believe the Stormex comes with a number plate window..? You could affix it to that (or affix to bike wherever the number plate window actually sits once cover on)?