Thanks for all the help so far.
So, I finally cracked the problem by going back to basics.
Essentially, these are the steps I took just in case anyone else ever has this problem:
1) I re-flashed the FC to 3.01. This stopped the constant beeping noise on the FC.
2) I revisited each tab on Betaflight and noticed it somehow reset all my chosen settings.
3) Receiver output was set to PPM. I changed this to Serial and used IBUS (I previously clicked SBUS).
4) Added the modes as it reset all of these too.
Voila - fully functioning RX and TX and all modes set.
I'll test out the quad tomorrow but so far, so good on the bench. It's responding well to arm, throttle, yaw etc.
Will give you guys an update in due course