Thank you for the information Wiebke - as always, you're very helpful.

After an initial panic of thinking that it was bumblefoot I then looked into spurs which seemed to look more like it! I certainly would hope that he wouldn't get bumblefoot - they are bedded on fleece and have a FULL clean and fleece washed every other day. However, at the moment I am on high alert and extremely paranoid because like I say we are currently looking into the possibility that Clegg may have a heart problem which I know bumblefoot can be a symptom of. Clegg is 4 1/2 years old and unfortunately my vets aren't proving very useful! I have been in touch with Vedra of the Cambridge Cavy Trust as I have just enrolled onto the Rodent Health Advisor course to see if she knows of any cavy savvy vets or rodentologists in the Wiltshire/Dorset area but unfortunately the vet's name that she came back to me with, sadly passed away. So at the moment I am feeling very useless and helpless and just want to do the very best for my boy.
As to the spurs, I'll leave them be but keep on eye on them and only trim if necessary.