So to help any one in the future.
There are small tangs as shown below with a square nut held captive which provides the location for the bolts.
The missing nut is in red, as everything rusts the nuts just break away.
The way I sorted this was to cut the shank of the bolt as close to the frame as possible then pull the tang out.
I then made 4 L brackets as below which can be held in place whilst the bolts are fixed.
The L brackets are a simple 12mm * 6mm Bar , cut and drilled. A SS nut was then welded on the end then through tapped. The Bracket was then bent to shape and powder coated.
YES I do need to clean the underside.