Joined: Mar. 2007
||Posted: 07 Sep. 2017,22:07
If the 20mm flags are intended for any areas that the cars will touch then they're too thin. The setts will be fine.
As a general rule, when clients supply the materials then I (and many others) have to charge more for labour since we need to make more than our wage alone to cover repairs and renewals etc. You'll be able to guarantee the product is the one you want but it's unlikely you'll save any money.
Best of luck with whatever you go for :-)
Can't see it from my house