Oh wow that seems to have fixed it.
After renaming the folder I loaded up DCS. After logging in again I had a default install. I was able to use the radio properly using the cockpit PTT button, the keyboard hotkey and the communications menu hotkey.
I then deleted the new folder and renamed my original DCS.openbeta back to it's original name and then loaded back into DCS. I was back in my usual setup with my usual hotkeys, etc. When loading into the mission I was again able to use the radio normally using cockpit PTT button, keyboard PTT hotkey and communications menu. I have no idea what changed but I'm guessing it reset some setting somewhere that was out of whack.
Also, I just realised I do have SRS radio installed though I didn't have either the server or client app running either before when things weren't working or now when they are. I'm not sure if that's relevant but just thought I'd let you know.
At any rate, thank you very much for your help in getting things working again. If I can send you any files etc to help you work out whether this is a genuine bug or just something messed up on my install please let me know.