If anyone else is having this issue I have found the problem - it’s the VPN! Switching it off resolves the issue. I could cry.

I have no idea why, but my VPN (provided through Norton) prevents the game from connecting. I’ve had a VPN for years and never had an issue so I couldn’t say why it’s suddenly become a problem.

Thanks everyone for your suggestions and I hope this is useful to someone else at some point!