Thanks Guys, I found the bracket!
It was in the front luggage compartment!
I picked up the car at the weekend, a number of parts have been stripped off of it. I'm not familiar with the bis engine so I didn't recognise the bracket at first, but the technical download section helped me to identify it.
Incidentally its a weber 30 DGF. The top thread for the airbox mount is badly damaged, but I'm awaiting a couple of scrap/spares 30DGF bis carbs, so I should be able to make it right.
I will have to start a restoration page on the car. Lots of rust, but very low mileage.....