Hi, Dalek.
I concur with the advice the other forum members have offered. I must ask how you know your mid-flower problem is pH related?
And, if you knew about the pH problem, what did you do to correct it? What was the suspected pH reading? (I assume you had a good pH pen!)
Let's also go back to your decision to grow in soil. Why that decision? What kind of soil?
I see newer growers make all kinds of set up mistakes, soil or grow media selection is but one.
IMO, I would swap out your soil for 70/30 or 50/50 coco/perlite, use GH nutes plus CaliMagic (follow Nebula's mixing schedule, not the bottles') and try again. I have never had an unsuccessful plant out of three or four dozen grown using the above. If you do not have a pH pen, you will need to get one or, at least re-calibrate your existing one. Essential for mixing nutes and testing runoff!
Have you researched or even read the many fine grow articles here on GWE? That is my best suggestion to ensure a great harvest.
Regards.