I managed to have a look at my garratt yesterday. I found a piece of
lead flashing (about 1.5mm thick) in the garage. I cut 2 pieces 55 x
12mm to lay between the driving wheels under the keeper plate. I
removed the centre screw from the keeper plate and used it to hold the
lead in place.
After a test run, it didn't do much better. So I added another layer of
lead. These pieces were 60 x 12mm and were attached with the same
method after about 20 minutes trying to find a longer screw (turned out
to be a 9BA x 3/16 long).
This time the engine hauled another 200g of rollingstock (4 more RU's
or two VLX vans).
The double layer of lead adds 55g of
weight to the loco. Total weight
of engine now 785g.

For the record -
RU's weigh 50g model (35t real)
OHG weigh 85g model (20t real?)
Test train 1 (no weights) - 13 RU's + OHG = 735g (475t)
Test train 2 (with weights) - 17 RU's + OHG = 935g (615t)
Gradient on my spiral is 1:50, with a radius of 700mm (29in) - should
give me a grade of 1:40 with a bit of creative maths.