Grub screw in mixer tap?

[Brijmundo]

Hi, I'm thinking I need to change the cartridge in the bathroom mixer tap. It's very stiff and only runs hot water. I've located the grub screw (I think) but can't make out what sort of key it needs. It appears to be circular as a opposed to e.g. hexagonal. I've tried a couple of allen keys with no luck. Is it possible that it requires a specialist tool of some kind? It's a vitra mixer/lever tap. Thanks

 

[jj4091]

Torx screw? https://www.google.co.uk/?gfe_rd=cr&ei=TmjWU-yjCOnH8gfZnoD4AQ#q=torx+screw+bits

 

[foxhole]

Usually allen keys since when fitted access is restricted, you will need the correct size metric key, usually quite tiny, 1-2mm.
Any chance of a photo?

 

[Brijmundo]

think you may be right on the tiny tiny allen key suggestion