2001 Peugeot 307 Water Leak

We have a Peugeot 307 HDI which has a mystery problem.
9 different mechanics have looked at the car and the water pump has been changed but the warning light to top up water coolant keeps coming on every few miles.
There is pressure in the header tank even when the engine is cool and water is escaping from here when the engine is under pressue.
There is no foam or water in the oil, no white smoke from the exhaust so it can't be the head gasket and has been pressure tested so all seems fine in that area but STILL we have this problem. Do you have any clue as to what this could be please?