Think I’ve figured it out – had to toggle some switches in Speed Optimiser plugin to force SSL. Thanks again! Instructions here from my hosting provider, in case anyone else encounters this issue in future: https://my.siteground.com/support/tutorials/getting-started/add-ssl-site/#Configure_WordPress_to_Work_Through_HTTPS
That’s weird. Haven’t had any other issues. Should I raise with the hosting provider? Thanks for speedy response (and the awesome templates… donation incoming)