This may not help you too much because of the colour but I had quite a bit of scuffing on the blue leather seats of my MGB.
I found some leather reviving shoe polish of the same colour and rubbed it in and then brushed it off as if doing shoes.
Then polished it off very well with dusters until there was no colour coming off
Came up a treat.
I think you might have difficulty finding dark grey though.
Looks like you can get it on line though
http://www.timpson.co...hoe-polish/g/444/grey