Hi Paul,
The drugs he has given you are anti-inflammatory's, make sure you take the whole course.
Obviously I cannot say exactly what you have but it could be cartiledge or tendent problems.
If you participate in swimming without offending you your stroke technique might be causing you problems.
I tore my ligament in my knee 2 years ago and still suffer.
If its causing you really bad problems still ask the doc to refer you for MRI however the NHS dont like doing this so you might be better of paying provate (400 to 600)
Try taking some Glucosamine Sulphate 1000mg per day also, this repairs the cartelidge.