Seems simply a matter of buying one of their marinades, let the chicken soak in it for a while, then stick it on the barbie and brush again with marinade
their website tells you www.nandos.co.uk. Doesn't sound like rocket science to me - get chicken, marinate in peri-peri, cook on barbecue (butterfly cut means you slice it in half but retain a join on one side so when you open it up it looks like a butterfly)