Extenuating circumstances??

Basically im taking a gap year but want to apply for medicine for the 2015 entry. I have always had a leg length discrepancy but in the past year it got significantly worse (even teachers were asking why i was walking like I was since I've had operations to correct it). So because of this I had mild back pain which also got worse because of the continuous time i spent hunched behind a desk. At this point I didnt think it was anything big enough to get special consideration for. I thought it wouldnt affect my performance significantly with my a-levels, but it seems to have. I saw my gp (I did try and get in touch with my surgeon may/june time but he said he'd discharged me and I need to go through my gp again) in july and he said it looks like my leg length discrepancy has led to me getting scoliosis so he's arranged for me to get a 'heel' temporarily. He said he'd write a letter for me for extenuating circumstances, as proof that i've had to deal with back pain through my exams.
Will this be enough? will unis understand why i didnt speak to invigilators about this beforehand?
Thanks