Possibly has 2 immersion heater elements, lower one on energy7 the upper one to top up whenever at standard rate. would suspect the lower element or thermostat has gone, thermostat sits inside the element, thermostat is an easy replace (and easy check) couple of cables, element will require an empty tank and special spanner, worth getting someone in I would have thought.
parts not very expensive.
http://www.tlc-direct.co.uk/tcl/sear...&Submit=Search for an idea of parts, course they have to be the right parts.
Guess you would just replace both parts with new.