I've been ignoring the situation for a couple of weeks. I don't think it's worth getting any more done by the garage. I could do some limited investigation myself.
Oddly, I've heard nothing from the garage. I need to approach them. They quoted £936 to do the injectors and I trusted that this would fix my car and it hasn't. Is it reasonable for them to expect me to pay them this and take back a car that isn't fixed? What is the experience of people here of this kind of situation?