Firstly here is a copy of my email (in italic font) of thanks to Screwfix for a prompt refund and the helpful advice from this community. "We do not now need this thermostat and so will not be collecting it from Worthing Ordered the new immersion heater thermostat as our failed immersion heater, in an unvented tank, was installed professionally about a year ago . I thought the problem might have been overheating or reset problems as described on your community pages. Was disgusted to find that the “professional” installation had failed because the neutral wire had come out its terminal – there was scorching of the heat resistant sheath and the blue wire insulation was brittle and cracked. Again your community pages were helpful in describing the effects of temperature fluctuations on terminal tension! Cut out the faulty end of the wiring and reconnected all wiring tightly and all is now working well. Sorry for any inconvenience but would appreciate a refund for the uncollected thermostat. Thanks for all the support on your web pages dealing with immersion heater wiring and thermostats! Obviously our professional plumber was an amateur electrician! Like me he did not have the correct small size spanners to tighten the nuts on the terminals or the outer cover. Fortunately I have a needle nosed mole wrench that has done the job adequately without damaging the nuts. Do others agree that the design of our traditional immersion heater terminals could and should reduce the area of uinsulated wiring or posts and not rely on a somewhat flimsy over cover? My feeling is that it would be safer to have terminal blocks of adequate size to take the more substantial heat insulated wires needed for powerful immersion heaters. Provided the right size spanners are available is there an advantage to using nuts rather than screw down terminals? If we had experienced a fire and damage to property or injury to people would what appears to be an awful product design be responsible? Sadly the product seems so badly designed that it needs a professional electrician to complete a safe installaton.