i have a worcester 28i junior, wont turn off constantly using gas?

hi , my boiler has been working fine, coming on when i turn hot tap on or shower on, and firing up when i turn stat for heating,
my gas bill has been roughly the same over the last 12 months
but recently after this cold spell (not sure if it has anythink to do with it) i had a gas bill for 1 month and it was 4 times higher.
i then noticed that the boiler has been constanly burning gas when iv got the stat right down and no hot water running.
iv now resorted to turning the boiler off at the mains when not in use but this is just a temp fix,
does any 1 have any ideas what it might be?