According to the link below :
http://indefenceoffirstgreatwestern.blogspot.com/2007/08/reader-mail-vol-i.html

"Firstly, to answer Graham, who commented in my post on the DfT about the issue of trains from Westbury to Swindon on Sundays only proceeding in a northbound direction. The base Service Level Commitment set by the DfT requires First Great Western to run 2 services in a northbound direction only. This was based on analysis of passenger journeys and customer flows at the time. The DfT was not willing to commit to the investment that an additional southbound service would require."

"However, good news is that following our own analysis of passenger bahaviour, we will be operating a southbound service from December 2007. The added bonus to us of doing this is that we are able to run an additional passenger service on a unit that would otherwise run empty on its way to its next journey."

"Firstly, regarding Melksham trains. I can't give you an exact timetable of the December 2007 Melksham trains at this stage, but I am told that the service will be roughly the same, unless any last minute amendments are made."

"Various proposals were made to extend the Melksham service and restore the off peak trains. The most likey seemed to be an hourly off peak service. However, all the alternatives would have required external financial support. As we could not procure this, we are forced to continue running services in line with the Service Level Commitment. In addition to this, any enhanced service would rely on us getting extra rolling stock, which is not the easiest thing to do at the moment."

Lots of other interesting questions answered / pending , and my thanks go to the person who provided the answers.