Took some pictures of the sprocket.

IMG_20180501_195327.jpgIMG_20180501_195347.jpg

So it's a 10T and as you see from the chain picture it fits nicely no issues despite low diameter.

I will need to order the wheel sprocket soon as it is key to the layout. Once I know how it will fit together, I can work out the lowest angle for the wedge that I can get away with.