Apple Watch Series 4 Updating to OS6.0.1 - Time estimated says 6-10 hours then fails.

I'm trying to update my Series 4 Apple Watch to 6.0.1 on an iPhone XR. I've just updated my iPhone to 13.1.3 but when I try to install the latest watch software - it says the estimated time remaining is 1 day, then 12 hours, then 6 hours before eventually saying "Software Update Failed - An error occurred downloading the latest version of WatchOS on your Apple Watch".

Any ideas why / how to get around this?

I've tried restarting both devices and deleting the Update info from the "Usage" area on the Watch app.