You buy ONE of those little round adhesive red reflectors (about the size of a 2p). Cost a pound or two off ebay, then stick it ON the numberplate.
If you were that bothered about cosmetics, you can always remove it for the other 364 days of the year.
It has to be exactly central too! A couple of years ago my bike (different bike) failed the MOT as, although I DID have a little stick-on reflector on the rear numberplate, it was positioned in the bottom corner, technically a fail.