Last seen crossing the grass at Stowe, If no one has a spare would there be anyone who wants to go halves on a set of 4 from Junks?
Gave up with mine a couple of years ago after loosing the rears as well, found a couple of cheap copy ones for the rear but I have never found anything aftermarket that fits the fronts 
I recall ordering just a pair of these from hhc. The fix is to put a good spot of sealant on the retainng lugs before pushing into position.
Hi - I have a spare one that I can pop in the post to you. I have lost so many that I have lost count so I always keep a bunch spare. I have not lost a single one though since I have put a couple of blobs of sealant in!
Drop me a pm / email with your address and I will send it over to you.
Cheers Shiraz you have a PM
As per PM, just popped the wheel cap in the post.
Cheers.