Thank you for the introduction, C!

FYI, most of the complaints that come through the GMC are about serious untoward incidents - serious breaches of clinical judgement. They will all end up with the Primary Care Trust to investigate. I personally think the best route is to take it to the Complaints Department at the PCT - they will channel it through the practice's complaints system and then ask to see the response. If the response is not to your satisfaction then you can ask for a local resolution meeting and after that if you're still not happy you can take it to the Healthcare Commission and then the Ombudsman.
If it's something that you're unhappy about but you would rather get a quick solution to you should take it to the PCT's Patient Advice & Liaision Service.
I work in Complaints/PALS at a PCT....
