I have discovered a way to check if a car is Cat. C or D, without paying a fee or giving any personal information, such as email address etc.
Trying it on a few cars, it seems to be accurate.
First go to
http://www.autotrader.co.uk/sell-my-car
then key in the registration number of the car you want to check and the mileage.
Continuing should display the car details.
Then press "Next Step"
You will then see the following page
As you can see, on this example, the car is recorded as Cat. C under the car details.
Should the car be clear, then there will be nothing shown.
This obviously does not replace a full check, but might be of help for someone wanting to run a quick check.