I suspect this is not the problem because, as PhillipA says, half a litre in 400k is not a small leak. Nonetheless, I had a small leak of coolant and it turned out to be a small leak in the heater, not enough to leave a puddle, but enough to give a coolant whiff now and then. Only seemed to happen when the heater was operating so I thought it was the seal in the heater tap. Was only a small leak but annoying. Over time it has fixed itself.

