What is it with garages ragging the hell out of peoples cars?! My bog standard 1.4 Seat Ibiza went to a garage for a new timing belt / water pump, they picked it up and dropped it back to me in a courteous and timely manner, but my fuel had gone down loads in the space of the day and my mpg was sitting in the 20's (I can easily get 40 - 47mpg out of it, that's why I bought it) which tells tales of that poor little 1.4 engine being mistreated!
Although still unacceptable, you can sort of understand it if its a fast or sporty car, but not a standard 1.4!!! I'd be looking to name and shame if I were you mate.
That will have sat for ages idling making sure there was no air in the cooling system. They may have also disconnected the battery wiping the short term data. The road test, judging by my experience of most garages, would have been nothing more than a spin around the block or doing the lunch run to the local shop.
OP - I would mention the footage and your dissatisfaction to the garage and give them one shot at putting things right. If they fail I'd just expose them as much as you can. Owners clubs, Google reviews, Facebook, Twitter the lot. It may be harsh but they obviously have no respect for their customers so those customers deserve a heads up.