Hmm. If the hub is an AW or its ancestor, the circlip can easily be removed with a couple of very small straight-slot screwdrivers and a small hammer (drive one screwdriver gently between the circlip and the groove in which it lives near one end of the circlip, pry up the edge, and then use the other screwdriver to pry up the circlip, working your way around (rather like you remove a tire with tire levers); the circlip should pop off after you get about halfway around).

However...

While it's probably a good idea to take the hub fully apart to give it a good cleaning, you don't need to remove the cog in order to service the axle bearings (though you do need to remove it in order to service the freewheel bearings, I believe.

This is all contingent upon your hub's being an AW or substantially similar to one. With some research, you should be able to determine exactly what you have.

Be careful!!!!!

http://www.sturmey-archerheritage.com/history.php

http://www.hadland.me.uk/gear.html

http://www.hadland.me.uk/2tables.pdf (this can help you determine the exact model hub you have: look on p. 3 for markings to differentiate the era of the hub; p. 10 for hub shell dimensions associated with different models)

http://sheldonbrown.com/sturmey-archer.html

Looking forward to more!