Time to back pedal...
So whilst my TV tells me it is fully up to date and no new firmware is available, it turns out there is but you have to download it to a USB stick and put into the TV that way.
Latest firmware (2200.9) for my model is here:
And much to my surprise - it fixes the recording issue and now programs can be recorded whilst using apps (I tested iPlayer, Prime and Plex so far and all good).
So it's worth searching for your particular model on line and upgrading Firmware that way.
Hope that helps someone - this issue has driven me crazy for a year now!!
Thank you for following up on this, I found this thread when having the exact same issue, was very disappointed thinking that was that. Just thought I'd come back and take a look and now I'll do that update! What a relief!
I have a UE55MU9000 with the same issue. Verified I have the latest firmware via manual download. Any further ways to fix the "failed recording when using an app" issue? Thanks