jim wrote:
I
suppose an oil watchman type thing would do the job but they're
£60 minimum, and I don't need wireless comms, would prefer "off"
until "on for level check" to save power/batts.

The 6ft tall c.1200 litre tank is the other side of a wall from
where the gauge reading c/would be done, and it contains
water.

Essentially I want to know when the tank is being over depleted.
It should refill constantly but if there's a supply issue,
measuring (even roughly) the level in the tank is the easiest way
to detect that issue and deal with it promptly before the tank is
emptied.

Being able to check that it is full in the morning /last thing
would be sufficient I suppose...

Any ideas anyone?

TIA

A sensitive pressure gauge installed at the tap end should do the job by
measuring the head of water. Would only work when the tap is shut.

You "might" get enough adjustment range out of a washing machine water
level sensor. Older machines will probably have ones with adjustment
screws and you could add extra spring pressure to measure a few feet
rather than the usual few inches in a WM application.
Would work with battery, light and push to read on a KISS principle.