Hi Suzy
With these types of problems the best bet is to phone Motability and while on the phone to them ask them to ring the garage to see what the problem is and when will it be sorted.
Garages and dealers do pull the wool over our eyes but are a bit more honest with the pay master. That said, I know dealers are full of misleading tales of the unexpected theses days but damage to the car during delivery and the fact that spare parts are an issue with deliveries the dealer might be just telling you truth to the best of their knowledge so a Motability phone call is the best way to get a straight answer.