The AAIB report says:
The aircraft had last been filled with fuel two weeks before the accident flight and a total of fourteen flights had been made prior to the aircraft's departure from Shoreham on 2 April 2001. The investigation therefore attempted to reconcile fuel uplift receipts with flying hours flown to determine fuel remaining before the flight. Unfortunately a number of factors mitigated against this. During the '50 hour check', a week before the accident flight, extensive engine ground running had been carried out. The amount of fuel used during the ground runs could only be estimated. In addition there had been two errors in entering fuel uplift quantities which in total resulted in 10 USG less fuel on board than indicated by the technical log. However, this error was more than compensated for by an uplift of 26.5 USG that had been omitted. In addition the estimated fuel consumption used by different pilots operating the aircraft on apparently similar flight profiles varied significantly. Lastly the heater on the aircraft uses fuel only from the left wing tank at a rate of roughly one USG per hour. The heater had been used for about two hours on the flights immediately prior to the accident flight. It was therefore not possible to determine accurately the amount of fuel on board prior to the flight by reconciliation.
If the AAIB couldn't manage to determine the amount of fuel on board before the flight in a 5 month investigation, it would seem to be a reasonable defence that the pilot might have difficulty too.