Hi!
Not to be argumentative, but do you know it's a sacro iliac problem? If it's the physio's diagnosis and the treatment suggested hasn't worked then possibly it isn't a correct diagnosis.
Can you describe the pain a bit more? Location, what if anything triggers it particularly (uphill running, downhill running etc), severity.
I've had a buttock twinge for a year now, and I think gradually it's getting better with physio, which I only started a couple of months back.