"Cannot Update as you're not connected to the internet" - Watch Series 3 issues

[-westy-]

Hi guys,

Just bought a second hand Series 3 - however it doesn't seem to want to update - when pairing with my phone, the above message appears as it wants a mandatory update.

I've tried the usual reinstall the Watch App, factory reset phone and watch, blah blah - was wondering if anyone else had any solutions?

Equipment:

Apple Watch Series 3
iPhone 11 Pro, running iOS 15.0.2.

TIA