Dale,
If you are the registered keeper of the car, then you should receive a letter through the post within two weeks if your car has been caught on camera going over the speed limit. However, the letter will ask you to confirm whether you have been driving the car when the photograph was taken. All you have to do is say it was not you and provide proof that it was with the mechanic at that time.
Sometimes the pictures that are taken are clear enough to show the driver so if it doesn't look anything like you, you have nothing to worry about. One last thing, you may very well not receive any letters at all if no camera caught the mechanic.
