I think it's 3 months too from memory when I had the garage.
No customer issues but I was forever chasing insurance companies for recovery/storage money and I ended up scrapping a few of them or selling on as ongoing projects etc.
Wank situation to be in but my advice is to send him a professional letter asking politely when he intends to remove the car from your premises upon full payment otherwise you will have no option but to charge storage for the next 30 days at £20 per day and then sell/dispose of the vehicle to recover costs.
I can write you a good letter if you send me the facts via pm,it's my speciality!!!