Leaking Tap - Modern type

[ianlighting]

Sorry for asking a question which must have come around once or twice! I have a bathroom tape that operates with a single pull up lever that can be rotated for hot/cold. Its only 18 months old and drips. Having removed the top with a small allen key through where the handle screws in I can see a complex cartridge affair which fits very snugly. I can see it seems to be contained in a brass nut sort of thing which I could get a spanner around if only it wasnt in the tap body. There\\\'s very litle space to get anything around that nut - maybe only 2-3 mm and its 10mm down inside the body, so whats the trick? Is there a special tool? Many thanks for your help.
Ian

[ChrisR]

Box spanner?