Another strange cause. The ribbed trim panel on the tailgate had taken a knock at some time and broken the little securing spigot just above the lock. This was letting in any water that got behind the panel. The water then ran into the base of the tailgate and because in their infinite wisdom Peugeot put the drain holes for the tailgate INSIDE the boot, water ran straight onto the boot floor.
It was long-standing and I have a bit of minor rust to deal with on the edge of the tialgate. Luckily no rust in the boot, jsut a ruined sheet of sound deadening felt.
I may just have a watertight 205 now!!!