Seem to have everything working again....sort of.
My process, incase anyone else has similar issues:
I removed the car from myAudi app. I added the car back onto myAudi app, stopping at the final step where you log back into the cars MMI. I deleted myself as the key user within the cars MMI. This wasn't straightforward, as it kept failing saying there was a connection error. I found a workaround whereby you need to change your PIN number on your myAudi account. This forces a server update (so I'm told) which enables you to finally delete yourself as the key user in the car.
Logged back in as key user on the car. Plugged the car in but Octopus app was no longer reading it, so I deleted my 'device' from the app. Set up the device again through Octopus, whereby it does the test charge etc. Seemed to be working, but after half an hour of being plugged in the car was still charging, and the Octopus app was still saying 'A charging plan will be made shortly after you plug in'. It normally makes a charging plan within a couple of minutes of plugging the car in, so knew it wasn't quite right.
Unplugged the car, then plugged back in. Then again deleted the device from Octopus app and set it up again. This time the charge was halted within 30 seconds and the app stated a charging plan had been made.
Successfully charged last night through Octopus, with the app correctly showing the total charge amount in the 12 hour charge history.
The myAudi app still isn't 100%. Frequently getting a server error, but it does sometimes refresh correctly.
What a process, just to restore something that was working flawlessly until a few days ago...