Theme switching not working for anonymous users

Hi, is it possible there' s a problem with the caching in the latest installation?

The module is working like a dream for registered users, but won't switch themes for anonymous users. I have:
- set user permissions for anonymous use of 'switch theme' and system 'select different theme'
- ensured caching is disabled, as well as block cache
- disabled all non-essential modules
- tried with default, non-custom themes
- tried on a local and remote installation

I'm running the latest installation (6.1), with a zen custom theme.

This is a great module for 'accessible' sites, which is what I'm developing, so could really do with some help

many thanks

Wil