I spotted that today, checked if it had already been mentioned on the forum, and here it is!
They ship different number plate mounts according to the specific country. I've just received an email reply from them about those for the UK. There's a choice of two as follows:
178mm x 178mm (approx. 7" x 7")
180mm x 100mm (approx. 7" x 4")
Personally, I'd like to see one with a separate, integrated, tail light mount, like the one that Motone have been promising for a few months...I might just hang on.