I would concur with Andrew. I've used stick on plates for about 4 years with never so much as a second glance from plod or MOT inspector. If you are anywhere near Devon or Cornwall, then place called Trago Mills (a large "sell everything cheap" emporium for west country folk) sell the blanks for under 2 quid, and letters at 10p each IIRC. I recently bought three sets of replacement "parts" for my car, so I can now jet wash with inpunity. Mind you, I haven't lost one yet, and don't look likely to, it's just that a thread on here recently suggested that they might become illegal to use or sell in the near future, and, like a good boy scout, I want to be prepared.