Late as usual...
Fuel filter water sensor p/n is WKW500070, about £36 Genuine or half as much for an OEM version.
Measures conductivity so as long as the circuit remains open the dash light won't show up.
A bit of a pain as it tends to get over-screwed against the fuel filter causing the threaded plastic needle to crack which eventually gives way to fuel leaks. Same thing can happen to blind (plastic) caps so beware of heavy handed mechanics!
Eventually turned out being useful the one time I had crap diesel (water contamination) loaded (long trip -- no decent petrol stations available), as the dash light turned on & had me squirting a bit of water every day for about a week.
________________
Matt