It's an EGR (Exhaust gas recirculation) valve and the silver thing in the middle of the picture...



It basically recycles some of the exhaust gases to reduce NOX emissions but in doing do it collects thick oily sludge, which gums up the valve, and the inlet manifold, to the detriment of performance and fuel economy. Blanking it off (fit the blanking plate where the green mark on the EGR on the picture is) can actually improve performance and economy, and it will still pass an MoT just fine.