Warning water in the derv filter

Water builds up in a fuel tank due to outside air been drawn in the tank, replacing the fuel been burnt. This air contains water vapour especially when we live in a country where it rains 300 days a year. It's the same with filling stations and causes lots of problems when they empty their tank completely into vechicles.
Either you got a fill of diesel from a near empty filling station or alot of water has built up in your van fuel tank.
This has became a hugh problem with new plant machines and trucks, in that the manufacturers now recommend that the machine is refulled at the end of each day because this expells the air and water vapour.
Definitely check that a new fuel filter is done as part of your service. Mark the old one, some garages hate changing them because without the right gear it's impossible to bleed out all the air.
I would probably remove the drain plug on the bottom of the fuel tank and run off a couple of pints, the check the water content for peace of mind. It's a bloody messy job

Once I drained the water and let a half litre of derv run out the warning light went back to normal ,but I don't like the Idea of water in the filter so as I said before a service is due

Your quite right Pat, and that before you get into adblue, diesel particle filters, exhaust regeneration systems, etc. Modern diesel engines don't like green diesel because the colouring dye blocks the injection systems. Most truck haulage companies have got rid of their own fuel tank because firstly, risk of theft, and secondly the storage systems couldn't keep the diesel clean.
It will get to the stage where you will keep your fuel receipts to claim back the cost of a shagged engine.

Water in your diesel will mess up your injectors tips which is a costly service ,the best tip is to keep your tank topped up when parking up for a period especialy in winter,to reduce the space inside the tank for condensation to occur.