200tdi 90 wont turn off

As the title says, only way to stop the engine is to remove the wire to the pump solenoid. I have noticed that if you touch the spade to the terminal on the solenoid it has a slight spark even with the ignition off.

Things I have tried:
Removing wires from ignition barrel - still runs
removed ignition relay - still runs
Have also tried pressing brake peddle - this has no effect though i know in some cases this has stopped the engine.

So likely some sort of earth issue? Where on earth do I start to look now?

Thanks again.

Probably not an earthing issue as you seem to have a good power supply (and earth) at the solenoid because it is doing what it is supposed to (power at solenoid = engine runs, no power = engine off). Seem to remember something about duff light holders causing permanent power to the solenoid, I would try checking all your lights (in various combinations) to check they all work as they are supposed to . Does it have any form of immobiliser fitted?

Post 5 here

No it does not have an isolator. It is interesting that the headlight relay gets warm even though they haven't been on.
I haven't changed any bulbs or anything recently. It just did this out of the blue.

Update - removed the fuse for the aux/horn/foglight and its back to normal. So something is shorting somewhere. Bugger.