Usually you do not have to dismantle it. The instruction sent in the reply over is good, more info here:
http://www.britishtelephones.com/dial1.htm#d10
I would have tried to blow out the dirt, clean with a little brush and oil carefully. Do not use oil with silicone, it may cause problems later for the contact sets.
A thin watch, weapon, bike, sewing machine oil are usually OK, but you cant hardly get to little.
dsk