Forgot to update this. The physio diagnosed it as shoulder tendonitis. He said he could feel some scar tissue and gave it some ultra sound, massage and finally some stretching. There was a big difference that was apparent immediately after the session. I was told the cause was probably just going too heavy too much as I had previously spent a few weeks of very heavy low rep training at the gym.

Since then I've varied the rep ranges a lot in the gym, I have however found I get a niggling in the shoulder from time to time which lasts a few days then disappears again. No activity in particular seems to cause this and it is no where near as bad as it was prior to physio. I'm planning to go back for a follow up though.

What does concern me is that I still seem to get a lot of popping and cracking from the shoulder, at least more so than the other side.