Thanks for your reply. I have done as you suggested and created another account using a different email address. In this case I did receive the activation email and clicking on the link activated the account successfully. I am happy to use this account instead of the one I was having problems with.
Just to confirm. My account was manually activated. I was able to log in, I started to download the T-splines plugin for Rhino. Whilst that was downloading I thought I would see what else was on the site so browsed off the page I was on. It was then that I kept getting the "resend activation email" notice. When I clicked on that link I did not receive the activation email (in my inbox or in junk - but I guess it could have been stopped before it got to me) and I was told that my account was already activated. The software continued to download successfully.
Anyway I'm happy to use the other account which seems to be working fine but I thought I'd let you know what I had experienced.