There’s several possibilities. First, and probably least likely, since it’s restricted to one file (or set of files) is simple wifi interference . Next, it could be a file type mismatch on the Mixcloud server, where the file  is marked as shareable, but encoded in some codec that Sonos can’t read, and subsequently disconnects. This feels much more likely, IMHO, since you indicate it’s always the same file / show. Since the data never exists on a Sonos server, I’d be getting in touch with Mixcloud about this, and have them look in to both the codec it’s encoded in, and any DRM that may be included, but not ‘flagged’ properly for the Sonos system to read it. It might be easy to test this by playing the file outside of Sonos. I’d actually be somewhat concerned that a CS rep won’t actually understand what you’re telling them, but it’s the one explanation that works in my mind. 

You could potentially contact a Sonos CS rep, but I’m not sure they’d get it either, as it’s relatively a random thing, and if they’re just reading from a script, rather than ‘getting’ what your saying, they’ll not only be confused, but give you a bunch of potentially unhelpful suggestions. And, honestly, it’s extremely unlikely to be something Sonos could ‘fix’. 

Interesting, yeah I'll give MC a shout and see if they can solve it. The same show works fine via SoundCloud on Sonos, so that would suggest it's on the MC side.

Thanks for your input