When I took the battery out the tray was rusty and soaking wet, and the wiring was resting in it. But when I wiped it the rust all came away and it cleaned up quite nicely. I thought the water had come from the last time I filled her up, but when I looked closely I have a small leak in the washer bottle, so that wasn't helping. I initially thought that was the reason for the current drain, but it turned out to be a red herring. But glad I found it.
I'm the first to admit my wiring is 'ad-hoc' and really needs sorting out so I thought that might be the problem. So I stripped all the diagnostic wiring out.
No, another red herring.
So then it was time to check the circuit. By undoing the mounting of the isolation switch I was able to isolate the battery then bypass the switch by putting a multimeter across the terminals. And Bingo, there was a 160ma drain. That doesn't seem much, but in theory it should be zero, so needed investigating.
Now, I thought the only things that are live all the time are the 12V charging socket, which shouldn't take any current, the lights, the horn and the Voltmeter I had fitted, which should take zero current. But then I realised that because the horn, the voltmeter and the oil pressure gauge are on the same panel, I had also wired the oil pressure gauge to be permanently live as well and I had no idea what the current drain on that was. So I unplugged that and the drain dropped to 60ma, so that was a help. Then just to check I unplugged the voltmeter and the current drain dropped to zero. So my voltmeter does take a current drain of 60ma (cheap off E-Bay from China so shouldn't really be surprised).
So both rewired to become active only with the ignition.
I then wondered if that was why my previous battery had started losing charge. Luckily I hadn't taken it to the tip so for the time being I have put that back in to see if it holds charge. I'll know tomorrow afternoon.
So everything replaced, with a cloth underneath the washer bottle as a temporary measure. And the wiring tied up. And extra insulation added to the positive terminal of the battery just to make sure.