infinity - you have to be careful with groin pain it could be a number of causes and it could possibly originate in the hip. I'm not saying this is definatey it but its a possibility.
Ask your GP to refer you to dept of Sports Medicine and to be seen by the consultant specialist. The specialist may wish to send you for more diagnostic tests including an arthrogram and XrAY.
As you've had physio and and MRI has shown nothing I think this next step is more than justified.
I have been suffering from a condition called labral tear and femoroacetabular impingement. This started out as vague dull groin pain but after going through several hoops including a completely useless physio, ultrasound, MRI, a specialist eventually diagnosed FAI with labral tear having sent me for arthrogram and Xray. Don't want to unduly alarm you, and its incredibly frustrating and it could be other conditions/injuries, but you've got nowhere so far, time to step up assetiveness