Following on from yesterday's problem with the Android Wear timer I think I now have a solution. It came up while chatting with Mike McLoughlin about the issue.
I got to thinking that this problem felt like one that I've seen a number of times before with Google stuff. One thing that's rather common (in many cases for very obvious reasons -- you can't cover the whole world in one go) with Google is how they struggle to get languages and localisation right. This felt like it was a similar issue. Mike had reported that his watch appeared to be unaffected by the issue (I'm guessing he's on the latest version of Wear -- the conversation headed off in a different direction before that became necessary) so I checked what his language was on his phone. Turns out he was the same as me: British English.
So much for that idea.
But then he suggested switching to US English and back again.
Happy enough to apply a very Microsoft "turn it off and on again" approach to a Google device (really, all big tech companies really are the same and really do suffer the same issues) I switched to en-US on the phone and tried setting a timer in voice on the watch.
So then I switched back to en-GB on the phone and...
...it still worked!
I've tried setting timers in voice on the watch a few times since and it's yet to fail.
It would appear, as odd as it is, that this is the fix. Well, a fix.