Apple Watch won't erase

Hi all,

I bought a secondhand Apple Watch recently and the seller forgot to reset/unpair it from their phone. I called and they told me that they had removed the iCloud lock so all I had to do was factory reset it. I'm familiar with the process (not my first Watch) so I knew how to get to the erase all data option. My problem is that the watch is not actually erasing. When I select erase all and confirm it, the watch just reboots, then takes me back to the old lock screen (and I stupidly guessed too many times at the passcode so I'm locked out of there even if I got that)

Anybody else had any problems like this or know any advice? Every time I google it just brings me back to the same support pages telling me to do the one thing I know doesn't work.