Well success! mostly!
After trying different configurations, we realised that the problem was with the side light circuit - which of course stays on even when the headlights are on. Shorting across the headlight switch, so the sidelights stayed off showed no drop in battery. But, of course the rear lights are on the sidelight circuit.
After a bit of head scratching, we realised there are two wire into the switch for the side lights. one for the rear lights and one for the front lights.
This meant if the short was on one of those the other would work ok. So, if the short was on the fronts, and we cut that wire, then the rear lights would work, and we would have enough lights to drive in the dark!!!!
So with trpidation I cut one of the wires (the thinner of the two, which I thought most likely to be for the front lights) and tried again.
Success! we had working back lights, working front headlights. We do now have no working front sidelights, but that should be a relatively easy thing to rewire. So we will be able to make it down to the meet, even if it is getting dark.