Hello James McKinstry,
I see that you're having an issue when asking Siri to set a timer for you on your Apple Watch. I'd be happy to look into this with you.
I use my Apple Watch daily and can understand wanting it to work properly. I was able to test "Hey Siri, set a timer for 5 minutes" on my Apple Watch, also using watchOS 6, and it worked as expected. Let's have you take a look at the article below for help with Siri on your Apple Watch. Go through the steps listed under section titled "If Siri doesn't respond or asks you to try again", and then test to see if the issue persists.
Use Siri on your Apple Watch
If the issue persists, unpair your watch from your iPhone and then once you have paired it back again, test to see if the issue remains.
Unpair and erase your Apple Watch
Let me know how this goes and if anything changed or not.
Have a great day!