Craptcha

I find that when I'm writing/replying to a thread (perhaps because of some time-out), when I come to post my message, instead of accepting the post, I get sent to complete a captcha

I’m confused, are you talking about this forum or your own application? Because I never had to complete a Captcha to post in this forum! 

On this forum.  My piece-of-shyte application hasn't been excreted onto the world as yet.

I use a VPN (with a loose end onto the web), so that's why the IP address is likely more 'suspicious' to the likes of CloudFlare... but it lets me onto the forum, and will (more often than not) only kick in with the captcha after a while... frequently when I try to post a reply as an already signed-in user.

You could install the 'Privacy Pass' extension, you can earn points by surfing normal or do challanges to dismiss the captcha check everytime.