Yeah, £55 was never going to happen I'm afraid, the issue with an A8 is that when something fails it tends to be more expensive, but there are lots of redeeming features too!
From memory, I paid ~£200 per caliper/motor and ~£100 for the ECU and fitted myself, calipers were refurb form ECP and the ECU was a dealer item. Put new pads on also which wasn't really needed but at ~£40 made sense.
It maybe the messed it up, it maybe not their fault - in any case I hope you can get it put right with minimum fuss and expense and get back to enjoying the car
