Still a problem
As a result of advice received on UK site: I will be checking vacuum actuator on filler door by plugging.
Checking filler drain, tank breather and condition of connecting flexible between tank and filler. I will also replace filler cap.
I will post feedback here too!
Grateful for any other ideas.
230 TE (W124) 1989 with 153,000 miles on the clock - hoping for at least another 100K