Hello,This is known as clinical negligence in law whereby your GP medically misdiagnosed your condition.
As such, they should have refunded you for whatever you paid privately. I suggest you send the Practice a letter before action setting out the basis of your claim and give them until say 11 April 2014 to pay you the money failing which you may make a negligence claim against the GP.
If they still fail to pay up, you may make a claim online for the money at www.moneyclaim.gov.uk
on the grounds cited above.
Hope this helps