As I said earlier - this does not seem to be the kind of case that can be resolved without legal assistance. You could go the Trading Standards route but that will take time and may not resolve anything.
Many solicitors offer a free half hour - worth a look.
Another thought - I would be able to claim for legal advice on my home insurance policy... maybe you could too.
In the absence of written evidence - I would write down, to the best of my ability, the content and dates of the phone calls, and who you spoke to. The more factual these things look the better. They won't have any records to contradict you anyway, so good guesses will do.
Was this garage GREAT BRITISH ENGINES by any chance ? In Essex ? They have has my car for four weeks! This being the second time and they have told me the same story of it over fueling etc and with the 3rd party electrician! Did they ever fix your car?