Fiat Ducato Diesel Filter Warning light

Dawn F

Hi

Our motorhome has just had it's anual service, when we drove it back home the diesel warning light came on.  In the handbook it says we have water in the diesel.  I telephoned the garage and was told they didn't change the diesel filter as it was only a
half engine service?  They thought it was to tell us the diesel filter needed changing.  We went back to our storage facility and checked it this week, ran the engine for a good ten minutes and the light has not come back on.  Has anyone else had this problem? 
Any suggestions what we should do?