Call me sarcastic but there is no such thing as a 'free anything' as someone pointed out, I wouldn't be at all surprised if it failed on something questionable which you'd be encouraged to pay full whack to correct.
Best place to get an MOT done is your local council depot. They will do their own MOTs on their vehicles (bin lorries, gritters, vans etc) and by law they have to offer them to the public. You pay full price for them but you can be guaranteed that you will get an honest test as they offer no in-house repair services so there is no interest in them failing your car on a technicality just to get you to spend money with them to repair the car.