Flotsam

The driver's side rear seat belt on my Fabia Estate is stuck, with a good amount of slack, but will not move in or out. After a casual inspection it doesn't look easy to access the mechanism. Are there any suggestions of things I (a car novice) can try before taking it to a garage? Assuming I can't fix it myself, my main dealer can't look at it until September. Is there any reason not to use a local independent garage? Also, if the belt needs replacing should I get a new part, or will a genuine used one off ebay be safe enough?