Thanks for the ideas. I could build any one of the suggestions myself but I think I will probably buy a solution. Time is money and all that.
The tank is underground and it would be nice to know if there is enough water in it to make connecting the hose to it worthwhile It's mildly annoying to set up say a lawn sprinkler only to find the water runs out after five mins and I have to move the hose to a tap 30 yards away. Taking the lid off the tan to check first takes several mins and involves a screwdriver, mud etc The pump has an anti run-dry cuttout but I'd also like to avoid relying on that. I could fit a float switch but it would be nice if the level guage could do that aswell.
I quite like the
units that "peter" posted. They have a meter with both a wireless link and a serial port on it for quite a good price.