Hi Nigel,
I have not had a lot of experience testing web applications at all,
but at work I know some teams do testing with Selenium. Selenium can
produce C# code so I'm thinking it would be a nice fit for
Concordion.Net. Watin or WET might also be suitable alternatives.
PLease let me know how your Concordion.Net usage goes, I would like to
help support you if you run across any issues.
HTH