Having determined that there was some form of racking alongside the Wood Store (previous posting) I went back over the photos at BritainFromAbove.org.uk and recognised similar structures alongside the Museum Garage and the Experimental Shop.
Here is the arrangement alongside the Museum Garage (the white sided building) on our N gauge model layout.
It is really difficult to identify materials on the racks as most photos of the factory only give distant blurry views. Where we have found useful photos, like workshop interiors, it is still difficult to identify many of the vehicle components that are laid about. I realised that it does not really matter in N scale. All we need is some odd shaped lumps.
In the close up photo below we recognise mud guards and wheels left over from our vehicle modelling but the rest of it is bits of plastic sprue and spare plastic kit oddments. Some of it looks like castings and are painted silver whilst other parts are painted with 'red oxide' primer.
I can't tell from factory photos if materials were laid underneath the racks (I can't even be sure that the model racks are accurate to prototype) but there probably were to some extent.What I feel is important at this stage of the project is to populate all the yards with some people, vehicles and materials so that the model overall looks like a busy factory and then add more details later as and when the mood takes us.