Video courtesy of Youtube and credit to the author: Larcs Workshop So we noticed that the expansion bottle level switch was leaking when we did the service. I took it out and this is what we found . Level sensor (with ‘O’ ring):- https://www.brickwerks.co.uk/t3-parts… Level sensor ‘O’ ring only:-https://www.brickwerks.co.uk/t3-parts… 2020-07-04