However unlikely (it had crossed my mind as well), it could be a possibility but the AAIB most likely will have investigated such a scenario. Fuel in storage tanks, fuel hydrants and/or bowsers are also monitored for contamination and records are kept. If there would have been a contaminated batch of fuel originating from LHR, I'd suspect more aircraft would have been affected.
I know, it is quite speculative to say the AAIB will have investigated because none of their reports have confirmed that they did but knowing their thorough investigative track record, my guess is that they did.
Regarding the water alert, none of the AAIB reports sofar have confirmed the claim of a water alert for any of the flights referred to in these reports. I am sure that if it was an issue it would have surfaced in the most recent interim report.
Green-dot