Yesterday I ordered a 23" Immersion Heater c/w Thermostat from
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and it arrived a few minutes ago .The box it was in was marked with the correct part number but was in an opened state and contained a 27" immersion heater c/w thermostat . It's too long for the size of cylinder I have so I called them and they said they would send the correct one out and to retain or dispose of the other one ...so at least I have a spare thermostat .I suppose at a cost of £7.80 plus vat it's not worth asking a courier to pick it up .
Screwfix did something similar a while back as well..
Stuart