I'm totally stumped here!
Performing the call isn't an issue, it's what SharePoint is bringing back. I have a list with a DateTime field with real name 'Session_x0020_Date', and one entry is '30/03/2017 17:00'. when I simply query using:
http://[mysite]/_api/web/lists/getbytitle('[mylist]')/items?$Session_x0020_Status=%27Open%27
It returns within the xml - 2017-03-30T16:00:00Z
Other rows return fine, it simply returns one hour less. I've deleted the row and re-entered it, same happens! Is there some gremlin in the works I should know about?