my worcester R40HE boiler turning off issues?

the last 2 days this boiler that is just running the central heating will only turn off when the on off switch is turned .
neither the central heating timer or the room thermostat make no difference if they are on or off the boiler still stays on ?
Boiler serviced 6 months ago and is 2 years old.
I would be grateful for any help in solving this problem
Mark Matthews