Hi Alex, welcome from me in Hertfordshire, before you try anything else regarding your locks try squirting WD40 into your locks and any other orifice In and around the catches, I had the very same problem couple years ago, after a couple of hours it still didn't work so booked it into Main Agent, next morning it was working so cancelled M/A, still no problem after 2yrs. The WD40 did weep out down the sills a bit but wiped off no problem, shows just how much of it I used, worth a try at only a few quid a tin. If that fails then Tony's advice as always is sound as a pound being in the trade. Good luck. Rha, Roy.