As helpfully pointed out by thesweetcheat on TMA Forum, there have been some seemingly sudden changes to the 49 bus service from Swindon Bus Station to Avebury.
Anyone planning a bus trip to Avebury from Sunday 2nd April 2017, they now only run every two hours. Here are the times:
From Swindon Bus Station: 08.15, 10.15, 12.15, 14.15, 16.15, 18.15
Return from Avebury: 09.34, 11.34, 13.34, 15.34, 17.34, 19.34 (leaving Devizes at 11 minutes past the hour – every two hours).