Bad service

When in bought a thermostat fitted in my car nd the fan came on continously, I couldn't understand why until I went to the mac, took 5 min nd to realise the thermostat they sold me was faulty he put my old 1 back n the fan stopped he call the manager as I was next to the spare shop it was proven that the new 1 that was sold to me was faulty nd he agreed, it was not cheap now I'm waiting almost a month to get a refund Now iv been told by the manger that if I don't get a diagnosis report I cnt get my moneys back, wer how do I get 1 iv asked for a credit n no respond I'm looking to speak to the boss himself coz this is not exceptable, if any 1 knows his no please send me a msg as I will take this up further What pertice staff nd service given to me not the first time many time before