Blimey - I just saw the prices of 9-speed Campag lockrings on that site - £18.21 for cassettes with an 11 tooth high gear and £13.22 for 12/13 ...You've clearly not tried Google's shop search.... in stock here for one (Germany, but fast & reasonably priced shipping to UK).
http://www.bike-discount.de/shop/a15143/veloce-cassette-10-speed-13-29.html?lg=en&cr=GBP&cn=gb
According to Campagnolo.com, 13-29 cassettes still available in both Veloce and Centaur varieties.
As long as you get the right 'speed' (9/10/11, whatever) then all cassettes from a given manufacturer are interchangeable. I buy Veloce because they are affordable but use them on a more expensive Campag groupset. Cheaper cassettes are made of steel rather than lighter weight metals, but I think that steel lasts longer and the weight difference is small so I'm happy with them.Stu, thanks for the suggestion. I wonder if the veloce cassette is same size as centaur? Is the difference one of material used or dimension?