Car broke and garage tried to repair it. The quote for the repair was over £2000 (very poorly car!). While deconstructing the car engine to repair it the garage have racked up an expensive labour bill (approx. £1500 so far) the garage eventually realised 2 days in that the problem actually can't be fixed, which wasn't revealed until the engine had been deconstructed. We now need to pay to fix the actual problem, which is also very expensive, on top of the £1500. Garage doesn't want to waive the £1500 because they've put the man hours in... But also, we are now adding £1500 on to any further repair costs.
AIBU to complain and negotiate down the £1500?
YABU - they've put in the labour and because your car issue is so complex / engines are complicated, you ought to accept that that's just life... They did their best - you win some, you lose some!
YANBU - they should've got it right in the first place