Basically you can't tow more than 85% of the weight of your towing vehicle
85% is a recommendation, the manufacturer's max towing weight and the gross train weight (the total weight of car, trailer, occupants, luggage etc) are the legal limits - and in each case these relate to braked trailers.
On occasion the manufacturer's weights are far higher than 85% (a 1999 Freelander diesel has a max towing weight of 121% of the kerb weight) but in the case of a Mini with a max towing weight of 450kg* for a braked trailer, it's more like 65%.
However, an
unbraked trailer can't be more than
1/2 the towing vehicle's kerb weight - so you're looking at around
350kg for an unbraked trailer behind a Mini.
And that's before we get on to nose weights, which I think is always going to be this project's biggest problem - but I do sincerely wish dekz2uk well with it.
sources:
National Trailer and Towing AssociationDepartment for Transport - the horse's mouth
disclaimers:
* that's not a confirmed max towing weight, it's just what I remember, but an
unbraked trailer's limit of 50% of the kerb weight still stands...