Thanks for the replies. Sorry, I should have made it clear, I am looking for a silicon wheel as I fear that trueing a diamond wheel wouldn't be possible.
The angle of a cup wheel is too steep to fit in between the teeth of the cutter. What I am looking for is the wheel that is almost flat with a little dishing and a tapered edge.
I found one on Ebay but it was 150mm I need 125mm.