Hello Steve,
I think you are probably right regarding the likely hood of prosecution for an over loaded vehicle.
I am not sure why you started to relate my response to a limit of GTW 3000Kg or to the trailer MTPLM of 800Kg. I mentioned neither value! The trailer weight does not have to be reduced to 750Kg for compliance with cat B, only if the trailer is unbraked does the 750 MAM limit apply.
We know that Ben only has the a Cat B licence he told us, which is why I did refer to the weight of the caravan, because there is some confusion about the kerbweight of the car - which is not known. The figure we were given of 1720Kg may have been the cars MAM or even its GTW, in which case its Kbw could be substantially lower and even possibly below 800Kg, in which case the outfit would not comply with Bens Driving Licence limits.
And Lutz is also right, In terms of the driving licence restrictions, its not the actual weight of an outfit, it is the permitted maximum weights or MAM that have to be considered. So Ben could not use your argument of saying that the caravan weighs less than the car, but its MAM is greater.
I think it could be highly misleading to explain how you manage to comply with the law for your outfit, because you did not make it clear that you have the B+E category licence. The B+E limits are applied very differently to cat B limits, and your technique would be inadmissible for Ben.
With a little research I have found that a "Vauxhall Astra Estate (98-04) 1.6i Envoy 5d" has a manufactures Kbw of 1160Kg and a Max towed weight limit of 1050Kg. assuming this is either the same or very close to Ben's car his proposed caravan of MTPLM 800Kg is well within the capability of the car and also of his Driving licence.
Ben - Happy Towing.