Before calling in the heavy mob, you could try spirits of salts, which is the common name for hydrochloric acid. You can get it from hardware shops or builders' merchants. It comes in a red plastic bottle, ime.
'Tis evil stuff, though. Handle only wearing rubber gloves. Tip the entire bottle down the loo (gently, you don't want it splashing), close the lid as fast as you can (the stuff smokes), open the windows and leave it for an hour. Flush and hope for the best.
I was shown this by a plumber, btw. It is really really nasty stuff though, makes bleach look like lucozade.