however you can't be prosecuted specifically for 'too much weight' because
there isn't a weight limit.

For the vast overwhelming majority of cars sold in the last 30 years, the
manufacturers publish a limit. Just because that limit isn't totally
supported by specific legislation doesn't mean that you won't get done for
dangerous driving by exceeding it. The manufacturers have stated that, in
their opinion, the car is dangerous to drive above that weight. If you want
to defend yourself, you have to explain why the designers of the vehicle
know less about the safe towing limit than you do.

Also, I'd guess a 'DD40' conviction would be worse on your record than a
'CU50'. Getting insurance with a DD code on your licence must be pretty damn
hard.

Note that there are loads of things you may be prosecuted for whilst driving
that don't have a specific piece of legislation. They are just prosecuted
under a general careless or dangerous driving charge.

Christian.