The straps IIRC are pirelli webbing? On the MKI’s the 3 straps do hold tension, but most cars seem to only have the 2 outers doing their job (perhaps because they are no elastic enough and not being streched in the middle?)

I thnk they went to 2 straps on the MKII? the outer ones.. IIRC the header rail has a recess on it for the strap in the middle and points on the sides for it to attach.. did your MKII have 3 anchoring points on the header rail.. or 1 or 2?