Hi bryn.jones,
The Thunderbird/Lightning add-on was actually developed by a third-party developer. Unfortunately I'm not familiar enough with this add-on to be able to answer your question, but the developer may be able to help you. The add-on website is available at:

http://sites.google.com/site/tbirdrtm/

I'm sorry we're not able to provide support for third party apps. :(

However, if you're unable to subscribe to the iCalendar feed we provide (for example, using these instructions) would you be able to contact us so we can take a look?

Thanks!