Not sure how easy it would be to reject a car that is nearly 5 months old and been in an accident.
There's no time limit as to how long you have to reject a car;
If you reject in the first 30days you get a full refund.
If you've had the car more than 30 days but less than six months, you have to give the dealer one attempt to fix the fault before moving to reject the car. If the repair has not fixed the fault, you can reject the car. The dealer is allowed to mark the value down according to milage only, nothing else.
After 6 months, you can still reject to an unspecified time limit, however, there must be a proof trail the fault was there at delivery. Milage terms again apply in regards to dealers being allowed to mark down the value.
The accident shouldn't come into it, providing you have followed all the dealer & insurance repair recommendations, which in essence put the car back to 'new'/'as was' state.
Good luck