Here are the proposed December 2007 timetable specification changes , courtesy of Severn Tunnel Action Group (link below.)
http://www.saveseverntunnel.co.uk/FirstGW%202007%20Draft%20Timetable%20for%20comments.pdf

Lots of franchise documents can also be accessed from the link below.
http://www.saveseverntunnel.co.uk/issues.htm

A Bristol Evening Post article (link below) details some of the timetable changes , which include restoring the Cardiff - Taunton link.
http://thisisbristol.co.uk/displayNode.jsp?nodeId=145365&command=displayContent&sourceNode=145191&contentPK=17788602&folderPk=83726&pNodeId=144922

No changes proposed to either the Melksham or Severn Beach line specifications. One would have thought that FGW would have included these as a precaution , given the links below.
http://www.savethetrain.org.uk/forum/index.php?topic=2835.msg6078#msg6078

http://www.firstgreatwestern.info/coffeeshop/index.php?topic=41.msg60#msg60

According to the Bristol City Council Physical Environment Scrutiny Commission Severn Beach Line Update (03/04/2007) , agreement had been reached in principle with FGW to run a 40 min frequency train service between Bristol Temple Meads - Avonmouth on Monday to Saturday inclusive , and that discussions were required with South Gloucestershire Council to ensure that the Avonmouth - Severn Beach rail - replacement bus linked in with the trains.

The Severn Beach line specification currently requires an all - day Saturday train service to be provided to Severn Beach , which means that it would have to be altered to allow the 40 min frequency train service between Bristol Temple Meads - Avonmouth (with a bus link to Severn Beach) to run.