Disconnect the battery and wait until the clock on the car shows the same time as reality and quickly reconnect the battery.
That'll be plan B!
For those of us with a 4-seater and the battery hidden under one of the rear seats that would be unnecessarily hard work. No need for Plan B anyway; as others have said there is a simple push-in switch under the dashboard below the clock. But go slow as you approach BST because if you go beyond current time you have to go round 12 hours again.
I always think an analogue clock on the dashboard lends a small amount more class to a classic car. Mine is white-dialled and beautiful.
