Well, ever so near but...!
I managed to remove two out of the three failed clips that were still in the dashboard. The third was a child of dubious parentage. I had to grind the tip of the screw off and weaken the clip. Recovering them through the vent was doddle. Re fitting the vent was easy too after renewing some foam around the ductwork.
With the three clips out I bonded in three rivnuts using 2 pack JB weld repair plastic glue. So far so good.
I then cut and shaped three aluminium brackets from 30mm equal angle material and reduced the 90 degrees to something close to requirement.
With those brackets in place and nearly tight I bonded the binnacle to the angle brackets. If the perishing binnacle had not slipped a bit over lunch whilst the glue set everything would have been perfect.
So, after lunch I removed the binnacle (unscrewing) the brackets that were now bonded to the binnacle, and risked everything by trying to cut through the glue to re bond the binnacle in place where it should be. 
Unfortunately the perishing thing cracked (told you so I hear you say) but not too seriously so I had to overcome that by just renewing the glue and then re attaching the binnacle using some "P" section elastomer that I had in my spares bucket.
All in all it doesn't look too bad, it has cured the rattle and I would adopt this approach again if I had another spare binnacle.

All being well attached are two images. One shows the screwdriver assemble all stuck together with gaffer tape. Yes it is resting above the arch but that was only for demonstration purposes. It was just possible with the screwdriver between the arch and the top of the instruments.