how can i fix a small leak in my baxi 105e?

my baxi 105e has a small leak. it appears to be coming from a small hole in the centre of a nut (?). this is to the left of the lower internals. it sits centrally in front of a circular thing (about 100mil diameter) and behind a small switch type thing with two green wires going into it. whats happening? many thanks in advance.