You may have probably seem my latest post about making the facebook page more of a call to action page. I have a breif idea of what we can put on the page, but like you, I have not made one before. It'll bascially state the problem and what we have done so far.
Fault is definately injector, I couldn't chat with the garage as I was tied up at work, but he will find out if it is number 3 again and I could have it re-done for labour cost only.
Remember not to be passive in your approach, they tend to drop their guard and say too much after. I have managed to get a written letter saying they will not look into my case as I am not their customer (private owner)... I now have Trading Standards telling me Audi are liable for 6 years since purchase... Should this go to court, I got written proof!
I agree, we'll chase the watchdog next, they have to answer to SOMEONE...