Hi David
I think the bus timetables for Stagecoach are available on line, so perhaps Google Stagecoach Honiton or Exeter to Sidmouth and see if this coincides with your train timetables.
I know that the station in Exeter is not very close to the bus station - when I've done this before, I've got a taxi from train to bus station.
The bus from Exeter will go past the campsite - not sure where the stops are!
The bus from Honiton MIGHT come down on the other road. the bus "station" in Sidmouth is at the Triangle on Station Road. There's a bus service from the camp site to the town - details are in your working programme.
Good luck and I hope you enjoy the festival.
Derek