Quote ''One other thing - it seems to leak more when it is just sitting.

Might be short on the full evidence for this, but it was dripping a lot when I went and looked at it - car had been sitting since monday.

Went and got come coolant, put it in, still dripping. Drove a bit more, got the car fully warmed up, and now I went out and looked and there are no drips under the car''

I would expect this to be the water pump that leaks.

It leaks less when engine running and warm due to the temperature tightening clearances up on pump seals/shaft ??

I have seen similar on a Rover 414 but that pump leaked the other way ,only when engine was hot.