I would try and make sure you get the cash up front, OR (and most preferable) insist that he goes through his insurance, and you can go to them direct and deal with them and them only. You will also then be able to claim for a courtesy car (lease car like for like, so you are not put in some sh1tty Fiat Punto)
This has happened to be with my last Beamer, same amount of damage, car off the road for 8 days, but got a brand new Z4 to play with in that time (bill for that was £2000!) all paid for by Direct Line.
Your repair people will also be happier to deal with an insurer