1.8T mystery coolant loss

I know this topic has come up before, but i do want to try to eliminate the "easier" fixes befor replacing the water-pump blind.

The story is quite similar to the one is this thread :

I first noticed minor loss via the level in the expansion tank: about 250mL in 500 miles.  Car is parked on gravel so do leak spotted underneath.  Local garage could't see or find any leaks; pressure test, left pressurised overnight, hot & cold etc. 

However the loss is now much more rapid. This week I drove a short distance to a local town: round trip 9 miles. 

1. the car interior was hot from sun, so heater off

2. Engine cold, so thermostat closed.

I stopped after about 2 miles and noticed a slow drip from the oval aperture in the engine sub-frame. I continued the journey and when home (engine now hot) there was no drip.  After the car had cooled right down the loss of coolant was about 200ml (estimate).

All advice most welcome!