I tried all of the above solutions without any joy, however I found a solution that seems to have worked! I noticed there was a prompt that flashes up just after you accept the update (following the pair scan) - it's in the blink of an eye, but it prompts to 'Update Apple Watch' then disappears just before going to the update page. The solution: Once you hit the 'Update Now' button, keep tapping the middle of the screen towards the right (to hit 'Update') - it took me a few goes, but managed to get it in the end!