Any tips on getting the shine back on the black chequer plate on the van .I have previously siliconed the surface but only lasted around 6 weeks then back to dull.

Welcome to the Forum GBR.

The best way to reshine the chequer-plate is to have it resprayed Note silicon is painter's worst nightmare so ensure it is well cleaned with a "Prepsol" before spraying.

Black is a terrible colour as it is soft in the pigments and will dull very quickly. Your checkerplate is more than likely powdercoated so I would be careful about just spraying paint over the top. I would contact Dulux as they may be able to advise on what is best to do with it. They used to do a primer for powdercoated surfaces or maybe even a clear coat over the top, but get some expert advice.

I have heard that Penetrol aids adhesion of oil based paints. So far for me, no personal experience with Penetrol as a primer.