Parking fine on car left in care of garage

Hi all,

I received a fine in the post this morning for parking on double yellow lines in January. No previous knowledge of this. The letter states that this happened on a date in January when I had dropped my car into a garage and left it there to be fixed, the garage location is named on the letter.

Where do I go with this? The garage should have notified me that my car had received a notice but they didn't.

Should I just fill out the 'other driver' part and send it off or should I approach the garage?

Just to add, I didn't park it on the double yellow lines when I left it there, I drove it into the garage.