Changing a stuffing box is dead easy and they only cost a couple of quid. Once you have the water turned off and the 4 connections to the whole water section disconnected you can remove the whole section and it's only held up there by one grub screw!!! Here's the catch......the screw is most likely seized in because of the leak. It could be a timetaking drill and retap or 3 litres of WD40 and hours of patience to get it out. The later vaillant model you can just drill the grub screw out and replace the whole bracket but I'm not sure you'd get one for the sine 18.