Dave, Having seen this question discussed many times I am firmly of the opinion that there is no correct grey. Most vessels, especially the smaller ones, seemed to use whatever paint was available and so consequently many different shades could be found on the same ship.
I have an Envoy, a Resolve and a Navy Joffre with the Envoy and the Resolve sprayed with Halfords grey primer and the Joffre hand painted with Humbrol medium grey enamel. I actually prefer the Humbrol as I was able to generate some more realistic surface textures which then took washes more effectively. I used dark brown oil paint washes as well as a much more rust colour around appropriate areas.
I fully intend doing the same finish to the Envoy one day but it will be a long process as the Joffre took a couple of weeks.