BSA Mercury's have two adjustments for the trigger pull and one for the Sear, I can't remember which way round it is but if you look at the front of the trigger guard assembly you'll see a small allen screw and also one at the back, if the sear adjustment is tightend in too much this will be the cause of your problem...
the best thing you and do is Number one make sure its not cocked or like another poster said "Someone will get hurt", take the gun out of the stock undoing the two frony stock screws and the large one that runs through the bottom of stock at the grip, if you can take a pict