This started life as a plug-in countdown timer I got in a clearance sale at CPC.
After taking it apart and removing the 13-amp plug and socket parts I was left with a the guts of an immersion heater timer that can be set between 5 and 50 minutes in 5-minute steps . The relay was rated for 20A at 240V so it could certainly handle the heater current, so I set about blanking off the hole where the socket was and then glued the back of the timer to a standard blanking plate with Araldite. I drilled a hole through the middle of the blanking plate, added a 3-way terminal block and voilá, an immersion timer for less than £5 to compliment my £5 central heating timer. Now there is no danger of the immersion heater being left on for more than 50 minutes and if you need hot water for washing dishes that's just one press.