I think we can discount a gauge fault, as it "runs out" after 230 - 250 miles which, at 40-odd mpg is about 1/2 a tank - & it only takes 20 - 25 litres to fill it up, i.e. it is about 1/2 full.
That fuel weep is a new thing which will need a closer look - when I've got more time, more light (in terms of brightness and duration) & better access. However, I did start it up this evening & carefully examined the clear plastic pipe from the filter housing to the pump &, just as I expected, no bubbles! I then had a look under the tank with it running for any telltale signs. Must confess I couldn't identify what I described yesterday; did find about 2" of rubber pipe that was damp on the underside, which might suggest a pinhole or porosity, but I dried it off - & it stayed dry.
I think, having provided as much history of fuel issues, which I thought might mean something to those who know far more about such things than I,we have conflated 2 issues. I had no reason to believe there was an air leak problem now, it's just that whatever I did to cure that particular problem at the tail-end of last year, perhaps coincidentally, cured the earlier premature running-out issue; it is just that which has returned, but this time, at an even higher fuel level.
Sooooh, where does that leave us? Please don't say "taking the tank out" because that just isn't going to happen!