Quick question, when they quoted you for the ecu did they include the VAT?, a lot of parts suppliers dont and add it on at the point of quote as most of thier sales are to garages who are vat registered and that may explain the difference. I really dont think its fraud as you agreed to pay the price he asked, what he paid is irrelevent, i really think you would be laughed at if you took that to court. Up to you of course but id really think hard about all of this because you have involved other garages it could really muddy the waters when considering a claim.