Hi Scruv.
I used 1mm and ½mm thick styrene sheet.
First I cut the main discs, each 20mm in diameter from 1mm thick styrene.
Then I measured out more discs at 16mm diameter onn a sheet of ½mm styrene and drew lines that divided them up into eight parts. I taped this styrene down and drilled 3mm holes at the points shown in the image below.
Then I glued the two 20mm diameter discs together using scraps of styrene to make sure the wheels were exactly 7½mm thick.
I cut out the 16mm diameter discs and added them to the wheels then cut two more discs at 10mm diameter from ½mm thick styrene and added these to the out sides of the wheels - these represent the axles.
Next I filled in the wheel using Milliput.
Finally I added pre made bolts from Meng-Models
http://www.meng-model.com/index2_new.php?id=213