Hello Kevin, and welcome to the Society!
To the best of my knowledge there is no group currently centered on the Bristol / Bath area.
The closest for you I should imagine would be the new Wessex Group which has only just been set up, which wil cover Somerset, Dorset, Wiltshire and East Hampshire. The group is planning two meetings for 2012, one of which I know is going to be in the New Forest, though I have no information about the other. If you contact Wessex@ebps.org.uk
I am sure the organiser will be delighted to add you to their email list.
Alternatively, you could consider the South Wales and The Borders Group, one of whose meetings this year was in the Eastern Black Mountains.
There is also a National Meeting planned for 8-10 June 2012 in Wiltshire, preliminary details of which are in the Programme in the Autumn Mailing. If you have not received this, send me a private message and I will forward the pdf to you.
One thing that can be guaranteed is that if you do manage to attend at least one of the meetings, you will find that the members are very good at teaching fern identification to newcomers!