Fuel filter warning light

[j6393]

I have a 1998 2.5TDi and the fuel filter was changed recently.
But before that and now on a warm day (say 38 degrees or more) or after a few miles running the filter warning light comes on.
There doesn't appear to be any muck in the filter so I wondered if anyone else in hot areas had a similar problem.
Also noticed a couple of times that the light came on in sequence with the indicators - earth problem?
Thanks
Neil