I've had a similar issue before whilst repairing a boiler a few years ago - although not to the extent of £1300! After i misdiagnosed a faulty PCB for a fan I stumped up the cost of the PCB myself and charged no extra to the customer. As previously said it is a learning curve for the engineer, and with the amount of money it has cost him you can guarantee he will have learnt from it and he won't make the same mistakes again.
If you have agreed as a customer for him to continue changing parts at your expense though I don't think there's too much you can expect from him. I think some sort of agreement should be made for you to meet halfway as it's fault on both sides here by the sounds of it.