First the good news: Filled the ZT up and added a dose of Redex. Drove the car around locally selecting the routes with speed bumps. To London and back yesterday, watching the gauge fall to 1/4 at around 250 miles (the only time I've been wishing the gauge to plummet!).
Second the bad news: Started the car on this morning and the gauge indicated 1/2 tank.
Assuming no kind soul has added a few gallons in the night, I still have a problem. I've got hold of the clip (thanks Billy), so may be having a look if it persists. Is this likely to be the level sensors, or the pumps not all pushing the fuel around correctly?