Renault master loss of power, at times?

Renault Master 2.5 dci, 2006.
Over the last few days, I've had a loss of power, starts first time, no smoke from exhaust, the delay in power takes 3 to 5 minutes before kicking in, I assumed it was fuel problem, so I took it onto the motorway and foot down, once started it would run OK at 60 - 70 +, then the yellow heater warning light came on (indicating water in fuel filter or an electrical fault), with the light on the power was perfect even down to hill starts?????? but when light goes out (after leaving vehicles for a few hours to cool), the power loss started again, until the light come on???

Is this water in the filter? or a sensor problem? Any suggestions.