Hi Anthony, thanks for your post. On reading your summary it does seem that you might well have an insertion tendinopathy. You might also like to know that I had one on my left hamstring perhaps made worse by a lot of road cycling, which unfortunately ruptured whilst kitesurfing (I now have two screws in there and some rope-work!). So I 'feel' your pain. And it took me a good 18 months to recover.
Let's first consider the tendinopathy as this is mostly a chronic (long time coming) issue that is usually the result of a multifactorial combination of both intrinsic and extrinsic factors, each of which needs addressing. To passively stretch the hamstring isn't really going to assist an attachment which has been overworked (for a long time) OR that was injured at sometime and no longer coping with the load placed upon it. It's also a very poorly vascularised area which means that any healing potential is also delayed and often limited once the inflammatory response has resolved.
I would approach this from a biomechanical perspective. As a therapist would want to consider the 'why' is this area not functioning normally. Is something causing it to be overloaded. Is there an asymmetry perhaps in the pelvis that has altered the mechanics on one side, to the point where the hamstring muscle now cannot behave in a normal fashion, and therefore the load is transmitted to the pelvic origin/attachment. This could be likely and is a common issue. When looking at exercises to assist it is often the eccentric contraction or the timing before the end of the eccentric load and then following concentric shortening of the muscle that are most beneficial in the rehabilitation. So with that in mind you can work with your therapist to come up with some exercises to mimic the range of movement where the dysfunction is more symptomatic. This might include something like box stepping.
Hockey is often played on a hard surface so there are some limitations if that is your sport and footwear could help or hinder the mechanics and loads when playing on such surfaces. The potential usefulness of some orthotics could therefore be investigated. Deep tissue release work also never hurts and I am a fan of getting deep tissue massage to the entire system, from the upper back, lumbar spine and into the thighs and calf complex. Calf strength too maybe worth considering in the overall function of the lower leg to see if they are doing their bit.
I think the important thing to note here is that your symptoms are your guide to how effective the treatment approach is being. If it is static, i.e. nothing is changing, then the current approach is not addressing the issue. An increase in symptoms/discomfort can mean at least you are getting into the right area (but you need ensure you have adequate recovery time to see if that is progressive in the medium term). It will take some time (often many months) to resolve as it would have taken a long time to becoming painful. Ensure diet is more than adequate and you may even consider some supplements to assist general recovery and nutrition.
See if you can work out a multifactorial approach to this with your therapist. Biomechanics, range specific and function specific exercise, diet and load progression. You will then become much more connected to the things that are working for you and will gain more insight into the actual root cause. I hope this advice and commentary are of use and do keep us informed as to how you get on.