It is probably just the thermal switch which has blown. This is the part that sits on top of the heater tank and turns the elements on and off. You can replace this for approx £20. You can purchase this from National Shower Spares on 0845 126 4141. Delivery usually approx 2-3 days.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix plumbing.