Not heating cold water

this could be 2 possible symptoms 1 the thermostat which gets the heater element to work. 2 it is the heating element that is faulty if the element has gone then this is about £15 to buy which u get both thermostats which u might has well replace both at the same time now if its the heater element then this will be around £20-25 to replace now my best suggestion is call out an engineer that doesnt charge any call out fee this will tell u exactly which if these it is if its thermostat then do it yourself easy job which will only take u around 30 minutes to do from start to finish if its the heater element then get them to do it which should cost around £65 to do this is because it needs to be sealed properly otherwise it will leak and cause more problems in the long run