I have stress incontinence and have been sent to a women's health physiotherapist. I too have done the usual pelvic floor excercises but apparently these aren't enough. The first time I saw her, she took my history and gave me an exercise to do for a couple of weeks. The next time I had to have an internal examination to determine the strength of the muscleat the bladder opening and was then given a set of exercises, a specific scientific programme. I have been doing these for about 6 weeks and have noticed a difference. Obviously will be an ongoing, permanent thing but things looking good. Due for a review in a week.
By the way, the stopping and starting the flow of urine is not a good thing. It's ok to try once in a while to check the muscle strength but not frequently as it doesn't help the muscle.