Thanks for your help.
I think I’m going to follow in your footsteps and go for a crockersharper with india and arkansas stones.
It seems to me that the crockersharpener will allow you to move the graver in a circle if you wish, whereas a roller will only allow you to move in one direction which I guess is likely to create grooves in the stones.
I thought using the side of a grinder was a no no too. They are supposed to explode or something….