What would your advice be on the fastest method to learn to read notation competently? I can play quite well and have a good grasp of the theory behind it, but now I also want to be a good quick notation reader as well.

Doing a bit every day is the most common advice but it's slow work. I reckon breaking up the tasks into separate section helps speed things up - eg, practice rhythm notation with phone apps or drum books - staff note recognition with an app - practice learning the names of all the notes on fingerboard ,too - can be done as a mental exercise, away from guitar.