Garage mis-diagnosed problem (UK)

[staffop]

Recently my 3.0D sport wasn't starting, it turned over ok - fully charged battery but not firing into life. I got it towed to the local garage and they diagnosed it as the fuel pump. They didn't call me about it, just went and replaced it.
Got the car back yesterday and exactly the same problem happened today - not starting.
When I got the breakdown service out today the guy thought it might be a faulty injector. Looking on this forum at similar problems seems to suggest it is more likely the fuel rail pressure sensor or faulty injectors.
What are my options and claw back with the garage. They've replaced a part that didn't need replacing and I'm out of pocket by £270. Do I have any come back? Should they now sort the problem out for free?