Subject says it all, almost. I've been running get_iPlayer under Windows10 now, without incident, for a couple of weeks since the OS upgrade. Today, however, I open WEB PVR Manager, to find all my previously selected options are reset back to default. Newly chosen entries are appropriately actionned, but the "Save to default" is being ignored. 
Any pointers, folks, before I try a 2.94 re-instal?
Cheers all, and thanks in advance.

Al.

"Save as default" relies on persistent cookies.  Presumably either your browser settings or some third-party utility is wiping the cookies or preventing them from being persisted between browser sessions.

Spot on!
If I disable Firefox's Self-Destructing Cookies add-on, the problem disappears as you advise above. Many thanks for your help.