We had a similar issue and after literally weeks of investigation we found that we had a leak from one of the the toe-rails. Water was seeping in when it rained hard. In our 455GL we found that the water doesn’t make its way to the bilge and instead collects just aft of the keel in a space behind the underfloor storage. never more than about a litre but we keep the boat in the Med so might be different if we were in the UK. Removed and refit the toe rail and our problem stopped. Good luck…
The membership association for owners of Dufour Yachts