Got it figured
The co-ordinates need to be converted into radians first, because that's what format Excel expects the data to be in.
To convert your co-ordinates to radians multiply by Pi (in Excel you can use PI() ) and divide by 180. Do this for all of the co-ordinates, and then name these as LatA, LonA etc as before.
Ignore that first formula, use this one instead:
=MOD(ATAN2((COS(LatA)*SIN(LatB))-(SIN(LatA)*COS(LatB)*COS(LonB-LonA)), SIN(LonB-LonA)*COS(LatB)),2*PI())*180/PI()
The "*180/PI()" at the end converts the bearing from radians back to degrees.
Give it a try and let me know if it works for you!