First of all thanks to Olivious the Drummer who answered my query of how he had mounted the quad lock case to the sat nav bracket. I have simply followed his idea and added a little bit more peace of mind.
You will need a Quadlock case to suit your phone and these (come in pairs)https://www.ebay.co.uk/itm/Quad-Lock-Adhesive-Wall-Mount-Twin-Pack-Mount-Only/372643957980?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
Oblivious simply stuck on onto the sat nav bracket and whilst I am sure the adhesive is strong enough to to the job I wanted a "failsafe" as it were.
It would probably be possible to source a bolt and nut to do this properly, but the nut would neat to sit flush inside the mount for the case to fit. Being a tight git, I found another solution.
There is a hole slightly off centre in the sat nav bracket, remove the adhesive tape and align the mount with the hole, making sure the mount is also aligned so the case will sit straight, and stick it down. (I didn't and it is slightly misaligned, apparently I'm now also OCD.)
I got a small cable tie and checked that the fat end would not go all the way through the hole in the mount. I then made a loop with the tie, passing the loop end through the mount so it left a U underneath.
I passed two thicker cable ties though the sat nav plate holes at either side, then through the loop underneath and tightened all three up. This way if the adhesive should fail the cable ties will hold it.