I read on Will's blog that he had an unsuccessful visit to Bath library to research the Lansdown campaign. Somerset in the Civil War and Interregnum by David Underdown is worth tracking down and this page on the Civil War in the West by Dr John Wroughton is also good.
Read or download Hopton's (see pic from 1637) narrative online here
There was a skirmish at Claverton in this campaign - now the site of the American Museum. Two cavaliers were buried in the church from this engagement. Hopton wiki
Hopton came from Witham Friary just outside Frome which answers Will's question about the sources being there.