I have been having problems with my heating. A local engineer  came out had a look and diagnosed a problem with the central heating sensor. He obtained a new sensor and changed it for me at a cost of £85. However before he had even left the house the problem had recurred and was obviously not resolved. He then advised that the main board may be on the way out and advised getting an engineer from the manufacturer out to have a look

the engineer from thethe manufacturer has just been out at a cost of £250 and he diagnosed the same fault as the other engineer  When I said that this had been changed he checked and found the other engineer had changed the hot water sensor and had not touched the central heating sensor. 2nd engineer proceeded to change the correct sensor and heating now working fine, thankfully.

I have just called the first engineer  to let him know as he had wanted to know what the problem was and also to ask for the money back that I had paid him. He is reluctant to give me a refund. I can understand as he had been out to me and done some work on the boiler however I have had to pay a large sum to get the boiler fixed. I hate complaining and am not really sure where I stand. Am I entitled to a refund from the first engineer even though he came out? but made a mistake and changed the wrong sensor? He tried to tell me that the information was not in the manual but I know it was as the second engineer showed me the wiring diagrams.

I would say so - his job was to 1/ Identify the fault and 2/Repair that fault.

He only did part 1 of that job, part two he failed on so he owes you at LEAST a complete refund on the part (because the part he got and replaced was not necessary and was his error) and a partial refund on the labour (because he did correctly diagnose the fault).

I would set this out in writing - keep it factual and don't put any emotion into it at all. Be clear about what you are asking for and why, and then state that you will follow up with contact to trading standards and potentially small claims court if necessary.

If he is saying the info isn't in the manual that he had access to, and it is, then quote that also - all goes in your favour. He has messed up, and really, why should you have to pay the full price for that?

I'd take a photocopy of the diagram in the manual and put that in the letter too, that way he can't dispute it being there, also send it to be signed for on delivery so he can't say he's doesn't know anything about the letter.