What Size C Spanner For Shock Adjustment?

I'm planning to do some work on the rear suspension of my 02 DSSA Daytona and will need to adjust the preload. Does anybody know what size C spanner I will need for the two rings on top of the spring please? I can't get anything across the shock to measure them in situ at the mo, and can't find any reference online.

Thanks!

 

Thanks! Will take a look at the RS site

 

Thought I might go for these
https://www.amazon.co.uk/Martin-SHW...nements=p_72:419153031&rnid=419152031&sr=8-47