Depends on your hand controls do they have a indicator on the hand controls? Cause when wiring up someone could have caused a issue though having a indicator myself on hand controls they have never ever gone wrong touch wood. If not then I can not see how it effects it and they are just passing the buck. I would personally ask to reject it as not going into reverse sounds like a manufacturing fault and further down the line who knows what other faults might crop up. Talk to Motability about it and replacement car. They should offer courtesy car if not Motability will.