Thanks Darren, and there lies my problem.
Obviously if I order a 34910521C it could turn up as a 41mm Showa one.
I can't find any in the UK and am still plucking up the courage to call Moto Rapido to get the Ducati price.
I've found these:
https://www.carpimoto.it/en-GB/Bike_...rzocchi-40.htm
Which appear to be correct and worth taking a risk on.
Nasher.