Thanks for the detailed response Datcon. It was very informative.
The CAA expert went on about visual inspection of the tanks but then had to agree visual checks are useless on a Seneca unless the tanks were over half to three quarters full you can't see any fuel because the tanks are inboard and the filler caps out near the wing-tips.
How do other people check the fuel on these? Personally I would never be happy with a fuel level I couln't see. However I can imagine a situation where it was necessarry to have less than half tanks to keep weight within MTOW. Is relying on logs the only way then?