My first thought, Bob, is why this boiler does not have a programmer/timer already fitted?
The MT10 and the digital equivalent are fitted directly into the boiler. They control a bit more than simply interrupting the power supply. Without getting too technical, there are electrically operated valves for instance. All this is controlled from the circuit board.
Cutting the power would leave the boiler possibly halfway through a sequence of operations. Admittedly, a power cut would have the same effect, and we can't control them! Thankfully, power cuts are rare.
Worcester have a very good tech dept.. I'm willing to bet that they would not sanction continually interrupting the supply as a method of control. It would work, but I would inevitably fear for the life of the boiler. Do ring them to make sure.
The other thing is .......... it would be against current electrical regs to fit a normal 13 amp socket to a boiler supply. It will be protected by a 6 amp breaker or fuse. There is nothing to prevent anyone plugging the kettle into this. Ok, so the breaker would cut out, but it would still be very bad practice. There are little 5 amp round-pin sockets available for this (usually used for table lamps controlled from a single wall switch).
I'm all for thinking outside the box, Bob, but I really don't like this solution. £60 for an MT10 could work out a lot cheaper than future boiler problems.