Go back to the GP and ask for a referral, you are quite entitled to do that - or see a different GP in the surgery if there is one. You should not have to suffer this long without your pain being investigated. However NHS referrals can take a while - you can refer yourself to a private GP or consultant as sqad says, any treatment will probably be much quicker. You can have an initial consultation with a private clinician and be under no obligation to do anything else privately, just pay for that consultation. My OH saw a private consultant for a problem, the consultant gave him options that he would either operate privately or he could refer back to the GP to arrange treatment on the NHS. We opted for the first route and he was done in a matter of weeks, as against an anticipated 18 months on the NHS at that time.
The National Back Pain helpline might be useful to chat to
http://www.nationalbackpainhelpline.co.uk/
If you want to go private, ring your local BUPA or similar hospital and ask where the pain consultant works from, ring his secretary and ask for an appointment - it's very straightforward to do, and anyone is entitled to a second opinion.