Joes box, Finally a thread where I may be able to help! I have had the same issue for years and after seeing various specialists (massage, osteo, chiro, physio) I was recommended to a sports therapist for torn ankle ligament. During the treatment I told her about my hip and she gave me some exercises to do. I did them every day for two weeks. Within a week I was running 10 miles without pain. Now I do them nice a week and no return of the pain.
What you may have is a simple weakness of some of the muscles within the hip, which is exacerbated if you increase distance or start doing lots of hills. It feels like its deep within the joint and it's really uncomfortable.
The exercises involve living on your front, back or side and doing straight leg lifts, controlled up and down motion and holding the lift for two seconds. Also plank and reverse plank. Start with 10 reps and work to 20 and then 30 if you can. Should take you 20-30 minutes. Also work on your core. It's boring but it works. I was also advised to get a float and do lengths in your swimming pool with legs only, swapping between scissor kicks and frog kicks. I do about 20-30 lengths like this in a session and it's hard work.
I'm not a doctor, but this really worked for me. I'm doing my first ultra in 3 months and I couldn't have contemplated it before. And even if it's not the answer for you, you're not going to do any harm trying.
Also, if you're local to Surrey, I can put you in touch with the physio. Feel free to ask me any questions and let me know how you get on.