Hi Brendan,
In order to claim Self-Build Relief, they have to submit CIL Form 1 - Assumption of Liability, as well as CIL Form 7, Part 1, so yes, the person named in CIL Form 1 is the party from whom you need to recover the CIL Liability (Reg.54D). You can apply surcharges if you wish (20% of the CIL or £2500 - whichever is lowest), and must apply late payment interest (Reg78).
However, the regs do allow you to collect from the landowner (so long as every effort has been made to recover it from the liable party first). Reg.36 outlines what to do in the case of a default of liability.
I have a little sympathy for the new landowner if this occurs, but not much, as they have either chosen to ignore the Local Land Charge registered against the land, or foolishly bought without carrying out the required searches.
