Anyone got any suggestions on how to fit them to the sides of my pannier bags?
Hmm...
looking at mine, the back of the reflective bit has the fuzzy side of velcro on it. if you don't mind sticking something to the pannier, you could buy some of the sticky back velcro (often sold from rolls in haberdashery shops, or maybe hardware shops), and stick the hooky side to the pannier, and velcro the fuzzy bit to it.
or...
If it's like the one I have here, the other end of the strap has the fuzzy side one side and the hooky the other, so you can roll it round and it sticks to itself. If you have a handle or similar on the top of the pannier, you could attach the end to it, and let the strip dangle down. it might flap about and get turned over in the wind I suppose.
If your panniers can stand having a pin put through them (so probably not if they are waterprrof or valuable, you could even safety pin the strip on.