@Stephen
The photo is annotated with dimensions and sizes.
> Are there two large hexagonal head bolts, the same size as each other? What size are they please?
Yes same size. M13 and 110mm long.
> 1. Diameter of the thread or shaft of each bolt. (or an M size perhaps marked on the hexagonal head)?
13mm
> 2. Length from the underside of the hexagonal head to the end of the bolt?
110mm