A little more progress before work this morning, A quick test showed that it dosnt appear to need a central arch to support the brick work, the two arches along with the vertical at the bottom are enough to do the job on their own.

As a result I took a simpler option for adding the deck support, with a length of 80thou across the centre above the hight of the arch, and a triangular cross brace between the corner and the central spacer.

I am now debating whether or not I will use the same approach for the box either side of the arch, these have a lot more strength already so I am not sure the central support is required, instead an easier option would just be a singular triangular support between the corners to support the deck. I'm running out of plasticard now so that certainly does appeal.

As for the deck itself, I have two options, I can either make the deck out of a single length which is too narrow placed in the centre and then build the pavements up ontop of that filling in the gap either side (gap is about 10mm) or use two sheets which are the full width, but have a joint in the middle of the arch. (I dont really want to wait until I have more 80thou as while at the moment I am coming home from work wanting to do some modelling, I suspect that will change when I get back from Tignes next week