Hi when in a sitting position and trying to lift and cross my left leg (as if to put on a sock) I suffer pain in the inside upper leg and I am unable to lift the leg higher.
Not sure if I have pulled or torn a muscle, this has been like this for about 3 weeks.
Anyone know if I can exercise to strentghen or will this make matters worse.

It sounds like it could be a strained hamstring. You would want to get that checked out before embarking on your own exercise plan, just make sure what it is. More than likely you over exerted yourself recently and it will take time to heal, but see your Doctor to make sure.

Reference (http://www.sportsinjuryclinic.net/sport-injuries/thigh-pain)