Ok, have just checked Zig fitting insructions booklet and my assumption that the urf wire from the aux battery would go to the chassis was wrong (our urf problem was at the battery end) seems that it actually goes to terminal block 3 on the back of the Zig and then as you quite rightly say to an earth point on the tow car via the 12s plug & socket. So if the connections at the aux battery terminal and at the back of the Zig are ok then it does start to sound like the battery.