7 hours ago, Trusa said:

Have coolent leaking from expansion tank when engine warm mondeo 2010 1.8 d Zetec. Done thermostat coolent flush new tank and cap still backing up pressure

I would test if the pressure was building up before the coolant got fully hot. Release any pressure before a cold or cool start, Start and drive a few miles to start warming it up, until the heater is just starting to get delectably warm, say, then stop, feel the hoses and carefully release the cap. If it has built up appreciable pressure in that time, then it is most likely to be combustion gasses getting into the water from the cylinders, that is about the only source of pressure before the water gets hot. That would then suggest a head leak, though they are not common in this engine.

Garages can also use a chemical test to look for combustion gasses in the water, but it is not entirely reliable, especially if it relies on CO (monoxide), as a lean burn diesel makes very little CO except under heavy load.

Also double check the hoses to the coolant tank for any little cracks or damage, if that is where the water seems to be from.

Did you miss this advice from Dmc101... 

There is a restrictor in narrow bleed hose to coolant tank, replace hose or check restrictor isn't blocked. Had same problem on 2007 focus 1.8 tdci.