2006 Focus 1.8 TDCi losing water/coolant???

My Focus is losing alot of water/coolant (about 3 litres over 40 miles) once the engine warms up? No signs as to where it is coming from (wet weather isn't helping) and even garage is stuck for ideas? Thermostat has been replaced and had no effect, Head Gasket has been suggested but even garage is not convinced it's the head gasket as the engine is running too well and no signs of oil in water etc. Any suggestions???