Another thank you and a bit more info, which other users might find helpful:
- The return URL seems to work - but it doesn’t appear to need the +expInfo[‘participant’] part for Prolific
- Prolific seems to require a slightly different URL call - it likes to supply URL parameters itself, winding up with the following URL for my experiment: https://pavlovia.org/run/ccliftonma/relnouninterp_a_tryagain/html/?PROLIFIC_PID={{%PROLIFIC_PID%}}&STUDY_ID={{%STUDY_ID%}}&SESSION_ID={{%SESSION_ID%}}