It's an old SS and takes a sturmey Archer type sprocket. Is it held on by a circlip? There appears to be some sort of dust cap or cover screw with three indents. So how do I remove the sprocket to change it?

SA gearhub sprockets are held on only with the circlip. The three prongs are just spline guides I think.

"takes a sturmey archer style sprocket" WTF

pics needed

SA sprocket:

SA sprocket circlip

How do you remove the clip (and put it back on)?

flat bladed screw driver and a lexicon of swear words.

Use a punch to make sure it's reseated properly when you reinstall.

Same as the Alfine although SA sprockets are slightly too thick to fit.

Same as the Alfine although SA sprockets are slightly too thick to fit.

There appears to be 2 sizes of SA sprocket 1/8th and 3/32s. I've assumed my hub (not SA) takes 1/8th, but that's a guess and the new sprocket was only £3 so no great loss if it's wrong.

Thanks for the advice Stoner

banana – ive got some spare SA sprockets at home in the shed. If you find yours doesnt fit, I may have a 3/32 you can have gratis but I need to check w the verniers. Drop me a line if you need to.

EDIT: BTW dont forget you have the ability to adjust the chainline to some extent by flipping the sprocket over and using the pressing offset. (16T and larger anway – smaller ones arent dished)

Thanks Stoner, I have a 36t chainring, so I've ordered a 16t sprocket which might give the right SS ratio for my canal bank commute to work.