Thanks for replying :-) Pity. I wonder why? It seems crazy to have to hunt for a 3rd party app for this when Apple has two excellent apps that just need to talk to each other.
I did look at Motion and Fantastical, but whilst happy to pay for apps, I only buy from developers with a non-invasive privacy policy (so not Motion) and for this kind of thing I'd only pay a one-off fee. (I hate this push towards subscriptions; with the exception of essential privacy apps, I'm done with it).
Motion is especially ridiculous in that respect. It is (as you note) a bit rough - with an insane subscription cost and a totally unnacceptable 'privacy' policy: $34 per month, and despite being a paid app, they're data-scraping and selling user data: 'We may share, or we may permit third-party online advertising networks, social media companies, and other third-party services, to collect information about your use of our Site over time so that they may play or display ads that may be relevant to your interests on other websites or apps, or on other devices you may use.'
Sorted3 looks promising. (One-off fee and excellent Privacy policy). It syncs with Calendars for some aspects its own task list, but not all, so it's not quite there yet.