@MoJo Ah yeah it looks like you've run into this bug: https://github.com/NodeBB/NodeBB/issues/8515
You'll need to cherry-pick the referenced commit to fix it in your install.
I can post if I use this URL : http://sheffieldfitnessforum.co.uk:4567/
I can't post if I use this URL : http://sheffieldfitnessforum.co.uk/
@Matt-Worthington Seems to be working fine... so at the very least perhaps we can rule out a problem on the server end?
Just got home, and it's working just as expected. Must be something VirginMedia is doing, as they are the ISP we use at work.