Thanks for the response.

Went to a physio up the road yesterday, after a few prods and massages and stretches, she said she believes its damaged ligaments in my Wikipedia reference-linksacroiliac joint. Basically she said it can come from nothing, and that ideally I would need to rest for a long time, but as walking is part of everyday life, the joint and ligaments are constantly working, therefore will take longer to heal (as opposed to an ankle injury,say).

Exercises to do are:

Laying down on back, place a soft tennis ball directly under where the pain is (in my case, its my left arse cheek, near my tailbone. Knees at an angle, move the leg over to the side of the body so that the tennis ball is simulating a deep tissue massage. Repeat 20 times, but only once every other day.

Laying on back, lift left knee up, use left hand to move left knee toward right shoulder, and use right hand on left ankle to bring towards your body. this stretches the bumcheek. Hold for 10 secs and repeat 10 times twice a day.

This next one i found pretty hard because of the injury. Lay flat on back. Tilt your pelvis up and towards the top of your body. Only tilt until you can feel uncomfortable, never past it.

Hope this advice is helpful to those who have a similar injury, its so frustrating as the rest of my body is fine yet i can't even run!