The warning light just means there is too much water in the diesel filter under the bonnet. Diesel apparently quite often gets water in it and the filter is there to, well, filter it out.
I've never quite learned how to deal with this, but the theory is you open a little nylon valve at the bottom of the filter (yes water is heavier) and push the pumper thing until you have expelled the water from the filter into a container.
On my 405 the little nylon valve is hell to get at and it's best to unbolt the filter mounting by the battery so you can raise it a bit.
My garage just changes the filter assembly and cleans it ready for the next customer. In france you can buy them in the hypermarkets.
The warning light sensor on the filter can become faulty and costs £60 (must be at least £1.50 to make) so it's worth plugging and unplugging the connector a few times in case you just have a bad contact.
Cheers, A