After many years of umming and arring, I got my finger (and mulit cutter) out and installed a tail tidy.
In hindsight, I would have purchased a less 'universal' tail tidy so the number plate light would have tucked up tighter to the rear light. The universal tail tidy had enough adjustment to get the reflector over the rear wheel, but as our rear ends project out so far, that's not an issue.