The legality of your use of trade plates is, at least, questionable. To read the rules, see here:
http://www.dvla.gov.uk/media/pdf/leaflets/vtl3 01x1%20trade%20licence%20guidance.pdf
However, I worked as a trade plater for 2� years and drove many thousands of miles on them, without ever being stopped or questioned. We used to deliver 'end of contract' vehicles to auction sites and it was normal practice to collect the last vehicle of the week on a Friday afternoon, take it home, and deliver it on Monday morning. Many (possibly most) of the firm's 250 drivers would 'accidentally' forget to fill in the collection mileage of the weekend vehicle and then use it, on trade plates, for their personal use at the weekend. Nobody ever got stopped, even though they might have their wife and kids in the car (making it fairly obvious that it wasn't a legitimate use of trade plates).
I'm not condoning such activities but it does serve to show that the chance of you being stopped is almost zero. (In the next job I did, I worked alongside a retired traffic cop. He told me that he'd never stopped anyone for suspect use of trade plates and he'd never heard of it happening).
Chris