We just missed having to pay it by a few weeks with our planning application, so saved around £12K IIRC. Our planning officer mentioned it to me when our application was validated, saying we were lucky, and up until then I'd not heard of this levy at all. That's the only reason I have that document I posted above, because I went digging around to find what the planning officer was going on about.
I have just done some research on this as I have always been under the impression that the charges had to be paid before commencement of the build.
This particular plan was passed spring 2014 and I am under the impression that S106 wasn't being charged at that date as they didn't have a requirement at that time to raise money. In any case S106 was finished by May 2015 when CIL was introduced in Wiltshire. There is total relief for CIL in Wiltshire for self build. Also CIL at least is normally paid on before work starts. This is good news because if we do buy the plot we will probably submit new plans.