Well that feels slightly foolish, thank you!
Is needing to export as interactive for form elements to work a new thing? I'm aware that there are different intents when exporting but I thought that Interactive was for presentation-type documents with transitions and such. I was sure I've exported print documents that also preserved form fields before now.
Now, of course, there's the issue of Acrobat not respecting the appearance settings of radio buttons applied in InDesign but I'm not sure that's a fight I can be bothered with at the moment!
Exporting to interactive PDF has been there from InDesign CS6 when the form facility was introduced to InDesign. You can fine-tune your form subsequently in Acrobat Pro. You also need to Reader enable the form in Acrobat so users using Acrobat Reader can save the data they've entered.
If you intend to produce forms regularly it's worth getting some training on the strengths and limitations of PDF forms, for example It's okay to say no to interactive PDF
Useful info. Thanks.