I have worked on and improved my glass since last time and even added some water to it:
The glass on the left has 'water' in it, and the glass on the right is an empty glass.
I am still however, having some weird patches at the bottom of both glasses. I suspect this must be because the bottom of the glass concaves upwards slightly (I was advised to not have flat glasses) and I've missed a step somewhere or have unsuitable values in my nodes.
The glass with the liquid also has light interacting with it weirdly. What parameters should I modify/change to improve it?
I have attached the blend file below if you'd like to investigate: