electrical question

stephen25uk

we are planning on movig some stud walls around in our hoese as we have 2 very big bedrooms and a not so big bedroom we are goin to make the all the same size..roughly,i am a carpenter to trade so i have no pronlem doing this..however,i am going to remove a cupboard and move the hot water tank which is an electric immersion heater,i am wondeing how do i go about moving the switch that on the wall,i am ok rewiring the lead from the switch to the tank,but i know the switch is wired directly to the consumer unit with its own breaker and looking at it there is not enough cable to move the switch to where i want it,is there any way od adding more cable to it using some kind of juction box as rerwiring all the back to the consumer unit isnt an option