So I decided I fancied some of these -
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=220664847704&ssPageName=STRK:MEWNX:IT
I already have them in the front of the Clio but wanted them for the number plate, and for the number place on the CR-V. Checked the bulb fitment on the CR-V and found that it was a 501, just like everyone said it was for the Clio, so didn't bother checking the Clio. Ordered Sunday night, according to the tracking info they attempted delivery yesterday and were at the collection office.
Left work early, rushed to the station, train, taxi to collection office, got home, and found that the Clio doesn't have 501's in the number plate but actually has C5W's, which are long and cylindrical with a contact at each end. So while I was able to fit some to the CR-V, I was nonetheless a bit miffed at they weren't right for the Clio.
So, while some Clios might have 501's in the number plate lights, some won't. Also according to HIDS-Direct the correct fitment for the 05on Clio is C5W.
Moral of the story - check your car before you buy.