Topped up the oil today before our trip to Wales, then this started dripping...

Have I overfilled it and this is an overflow? The pipe looks like it should go somewhere as the end is not cleanly cut.
But I've not noticed it before.
Haynes manual is surprisingly not helpful. We are now on the road and I want peace of mind.
A quick reply would be much appreciated so I can judge how many bottles of oil I need to buy.
P.s. It sits just behind the bellows thermostat.