There are many Lowdens (and he was related to most of them) but only one Lowden with the initals JW which stands for Joseph William. I am certain that my granddad is the person on both of the medal rolls.
When he arrived in Sth Africa he was immediately deployed to the Kimberley Town Guard which was made up of soldiers from many different battalions as well as sivilians. He later rejoined the 10th Hussars following the Relief of Kimberley.
I have found the answer to my question on the Anglo Boer War web site, thank you QSAMike, where it states in the description of the medals and clasps...
"In addition, it was noted that:
1. No individual can have both the defence and relief clasps for either Kimberley, Mafeking, or Ladysmith."
Thank you all for your help. Cheers.