ok I've managed to get it all working correctly (following from my earlier comment)
I went through the checklist posted by Apple Engineer above
the TechNote checklist at:
TN3186: Troubleshooting In-App Purchases availability in the sandbox | Apple Developer Documentation
The thing that was blocking the products being returned in array was having not filled in ALL of the details needed on the AppstoreConnect > Business page
Namely payment agreement, bank account, tax status statements etc.
After I had done that, about 2 hours later, all my test calls and purchasing views were working and the whole purchase flow as usual for an iOS device
See attached for Sandbox confirmation
Apparently all "Business" info is needed even for Sabdbox transactions.
HTH folks. Shout if I can add any more info
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect