The nice people at my hosting helped me fixed this, Krystal. As I thought it might be an issue at their end but turns out it wasn’t. Most other providers wouldn’t of bothered fixing this as it was my issue, not theirs.

It was a rogue plugin. Rename the plugin folder on your WordPress via FTP or Cpanel. Then you should be able to get into admin. Rename plugin back to what it was which should bring back all plugins but they will be disabled. Then turn them on, one by one, till you find the one causing the issue.

Bad Behaviour was the one causing this. I note it hasn’t been update for months and not tested with the version of WordPress I’m on so I’ve left it disabled.