The undertray is now secured to the car. I needed to drill 3 x 5mm holes at the front (red arrows in the image below and the other two holes were already drilled in the subframe (yellow arrows)
I used 6.3mm (self driilling) stitching bolts, these are used for stitching corrugated sheet cladding together for buildings work but there is no chance of them self drilling into the subframe so I predrilled with a 5mm pilot hole.
