Hi Dave
It is VERY unlikely to be the head gasket, failure of the HG is extremely rare on the V6, especially low mileage examples such as yours.
It sounds to me like a compenent is leaking - coolant loss, especially towards the rear passenger side, is likely to be:
* HBV (heater bypass valve) leaking. This is located near the bulkhead on the drivers side, but it usually drips onto the bellhousing and falls on the passenger side of the engine. This is about £25 to replace and held in by three clips - easy job.
* Something in the V of the engine - either:
- Thermostat housing leak
- oil cooler plate leak
- coolant bridge seals leaking
- split in hose near the rear of engine.
Is the cooling system over pressurising at all? Do you have normal levels of heat in the cabin? Do hot and cold controls work correctly?
Be aware that when you have a leak towards the back left of the engine - especially if it's the HBV (Which can "spray") - this can result in water being sprayed on electrical components and damaging them - the DIS pack which provides spark to the plugs is very suseptible.
To check for leaks in the Valley, you'll need to remove the plenum and inlet - not at all difficult. There is a plenum removal guide in the maintenance section - then the inlet consists of 6 bolts, and then the bridge below has LOTS of bolts !
Make sure you stuff rags in the valve holes, so you don't potentially drop anything down there and cause damage.
Check out that little lot, and let us know how you get on.