thanks again for the reply.
I have realised that I cannot easily make the site structures identical. I could 2 weeks ago, but now I've changed the way the mobile site works, I can no longer do this. This whole thing is really about improving the experience on iOS devices (and Android, which I get a lot less).
No I don't want to set cookies.
No I don't want users to have the choice of site.
Yes I do want to check User Agent on each visit.
The sites are very different. The mobile is specifically geared towards iOS and uses a different approach to displaying the content.