Thanks for the quick response. Here's the query.
The disc ID is for 'In The Lonely Hour' by Sam Smith.
I've done some more testing and looked at the request and response messages with a network monitor.
There is the single Get request and the response code is 403. Strangely though the HTTPPayloadLine message (which is in JSON format) says
"error": "Your requests are being throttled by MusicBrainz because the application you are using has not identified itself. Please update your application, and see http://musicbrainz.org/doc/XML_Web_Service/Rate_Limiting for more information
There is no closing quotes on the line. I haven't tries parsing it as JSON but suspect it would fail. Also the address doesn't exist on the MusicBrainz site apparently.
I don't see how throttling can be the problem. I sent in a single GET request and even when I am using my application normally it's far below the throttling limits.
I've also tried it with various settings in the GET message headers with the same 403 result every time.
I have also tried the query with IE 10. Sometimes it works and returns xml, sometimes JSON and sometimes it responds with 'service unavailable - your requests are exceeding the allowable rate limit' again with just a single GET request.