Just clarifying a few points.
Maths are not marked a common centre.
For non essay based subjects like the sciences all papers are not double marked. After the briefing session, markers will do a couple of hours of double marking. Then it changes over to single marking. Random samples from each group are check marked by the senior marker. This includes seven markers, which are still considered to be short answer by BOS.
However, markers discuss with pilot markers any papers that do not fit the normal marking scheme.
I would endorse what CEM has said about the integrity of the system as a marker as well.