I know that membury Services can be popular for some audaxers (it apears on a couple of calendar events).
Just to let anyone with DIYs or Perms planned to use it overnight on the 26th/27th, there are signs up on the M4 so you would ned to get there either before 8pm or after 6am. I presume that this is for Motorway resurfacing on the entry and exit slips so technically cyclists would be able to ride in as normal but proof of passage may not be possible.
