D'you know, I kept doing that when I was searching for the support page, too. Guess I'm just more used to doing stuff with Kingston products than Kensington!
http://www.amazon.co.uk/gp/product/B002Q42S4E?ref_=gb1h_img_m-4_5847_4d8f0d6d&smid=A3P5ROKL5A1OLE Hope you didn't pay too much, Gareth!
Nope - mine was £20.99 when I bought it back in February 2014. Good news: the monitor arm has now arrived. So, it took a lot of hassle, but I now have a replacement trackball of the right model and a £100 monitor arm for my trouble. Happy days!