Not having much time to investigate this since September I drove around with the sub tank warning light on and the sub tank half full and not operational.
I had been reading about locating and testing the sub tank control module behind the climate control dials, but hadn't got around to testing it.
However in the meantime I read a reply in a thread on Patrol 4x4 by Frosty1970 that said "Before anyone does anything, Firstly, run a new earth from the body to the chassis. This will solve 75% of subtank problems!!"
In another post by Chaz he included this detailed image of where to run the earth strap.
Last week I got around to running a cable the same as in the picture above.
I disconnected the battery to reset the sub tank and after almost half a dozen individual small trips the sub tank light hasn't come on again!
It all seems to be working again as it should.
As a side note, from just initial tests, the remote to unlock the doors has gone from working no more than 2 metres away (and sometimes not for a few presses right next to the vehicle) to working pretty reliably from about 14 metres away.
I have to admit this is a fantastic double result from this very simple mod.