We've been given an old Mamas & Papas car seat, suitable for 9 to 36 kg.
It doesn't have a five point harness, it has a strap that comes out of back of the seat part that on the diagram looks like it comes up the side of the child and across their front in a semi circle up to their shoulder, where it holds the seatbelt.
Thing is, this strap appears to do nothing at all! M&P no longer make the seat, and I can't find any tutorial videos online.
Any ideas?