Going spare, you was spot on the money. X9693 wheels (Class 156) are indeed the exact same as X9888 wheels (Class 153) After reading your post I e-mailed Hornby themselves to confirm if the wheel sets were of the same size. The response from Hornby was they don't know and therefore couldn't tell me, because they had neither wheels in physical stock to measure. and they don't keep the size of the wheels on the system. (which I found strange, but where Hornby the manufacturer was unable to help you yourself a hobbyist was) So, I took a chance on your recommendation. wheels arrived this morning and they're the same size. Thanks very much, I can now press ahead on finishing my kit build.