After spending a long time looking, the best out of the seriously sparse set of options appeared to be Visual Captcha (https://visualcaptcha.net). Unfortunately it seems it's no longer under active development.. Fingers crossed someone picks it up or something new comes along. A good, open, customisable, and privacy-respecting captcha open that developers deploy in place of ReCAPTCHA is desperately needed.
@r0bbie
Seems to be PHP, JS and Ruby only.