Just my 2p's worth.
I read the original AAIB report on this (G-OMAR, I think) at the time it came out, and it (or something related I read at the time) mentioned that the firm he rented the plane from used it for charter work, and since in that line you never know if you will get five fat blokes with a load of golfing gear turning up, you don't keep much fuel in the tanks. Especially in a Seneca. This renter was told to not return the plane with too much fuel in the tanks, and he did his best to do as he was told. He made some fuel miscalculations on top of that.
On a more general point, I did my whole PPL in Cessnas, and not once did anyone show me a physical fuel check. This would have involved bringing out a ladder. I was told to check the two documents back at the office and if they agree it must be OK. It was immediately obvious that they were filled in by the same person each time....
I have never flown a self fly hire plane with usable fuel gauges. I always filled the tanks right up (even if the CFI moaned about it, one does not get charged for time to/from the pumps) and knew that gave me say 4 hrs range, and if the trip would take 2 hrs that was fine. Then you didn't need working fuel gauges, dip the tanks, etc. I think the whole business of fuel calculations and conversions, as taught in the PPL, with that stupid ex-WW1 ex-Wehrmacht circular slide rule, is an accident looking for a place to happen.