No, you CAN'T use a Wickes one! The reason the original is expensive is because it's a low wattage per inch model. This basically means that there is more surface area for the heat to be released - and hence it is quieter in operation. If you use an ordinary immersion heater then the actual element part is shorter and it is hence a high wattage per inch one. It will sing like a canary when it's heating! You don't want that during the off peak period (early hours of the morning)! toptrish, with all due respect, please stick to your field, whatever it is, as it isn't anything to do with water heating.
Barry, February 2014