Look in your handbook for the maximum permitted tow hook weight to get an
idea of what the boot can cope with. You can go up slightly on that,
because
the moment arm is less, but not by much.

The maximum tow bar weight has very little to do with the rear axle maximum
weight. Almost no car will allow more than 100kg on the towbar (whilst 75kg
is more normal) whilst the majority will allow you to exceed that as useful
load on the rear axle. Remember that when towing with a 70kg noseweight, you
are still allowed to use the boot, provided you don't exceed MAM or GTW.

As a very rough estimate, you can fill the boot with half the useful load
(MAM-kerb weight) of the vehicle. However, if you are cutting it that fine,
then you should book an appointment at a public weigh station and check it.
You will then have a much better feel for how much you can take in one go,
which may be very useful for the future, or if you will need many trips.
Note that you can't be done for being overweight on a trip to the weigh
point, provided you don't take the **** and can justify your calculations.

Overloading vehicles is dangerous and you can get really quite severe
penalties for it, but the fact that you asked the question suggests that you
have the right attitude. I saw a complete nutter loading up in B&Q a few
weeks ago. He had a Ford Tourneo (small minibus transit van) that required
him to place almost the entire load behind the rear axle. A quick
calculation showed that he loaded over a tonne into that gap. I reckon he
exceed not only MAM by around 200kg, but the rear axle weight by a whopping
500kg. I seriously considered phoning the police, whilst this nutter thought
it was a great laugh trying to squeeze in one more dense concrete block or
bag of plaster. When it comes to building materials, vans and cars are
seriously weight limited, not volume.

Christian.