You need a pair of circlip pliers . These come in two types. You need the type where squeezing the handles together causes the points to move apart. Wear eye protection. Do not work in a cluttered area, or you will loose the clips. Use the pliers to open the clip just far enough to allow it to come off the end of the shaft. If you open the clip too far it will snap . Do the job inside a large transparent plastic bag. After removing the clip, the gear will just pull off the shaft.
Thank you for everyone's replies.
I've already got a pair of circlip pliers on order, as I remember needing one when I tried to remove the cog from my Cyclamatic clutch mechanism (picture)
Unfortunately, the cog didn't just pull off,and I'm certainly no weakling.
In the end, no amount of coaxing allowed it to come off cleanly, and I ended up destroying the bearing housing, prising it off with a screwdriver. Appeared to be made of soft cheese
Just wondered if there is such a tool available, suitable to help pull small cogs off.