The Mouton-Cadet is (or, at least, was) a reasonably decent drop of plonk. If it's been stored properly (on its side, with the base of the bottle slightly raised relative to the top, to ensure that the cork can't have dried out) there's a good chance that it remains highly quaffable. (1985 was good year for Bordeaux). It's not highly valuable though, simply because Mouton Cadet is produced in quite large quantities. (You can pick up a bottle of the recently produced stuff for a tenner in Morrisons). A quick bit of googling suggests a price of around £20 to £30 for the 1985 vintage.
I'd open it and expect it to be fairly similar to the £10 bottle which Morrison's can sell you (although possibly just a bit better) but I'd have another bottle of something on standby in case it turned out to be like vinegar.