I would suggest tar deposits coming through the brickwork/mortar. Has the chimney had a slow burning wood stove attached?
In my experience you won't be able to re-plaster without the stain coming through. - Hence the ali foil previously used as an insulator.
Strip back the plaster to see any brickwork/mortar failures and act according to the findings.
Tanking might be needed to prevent restaining

Thanks for your comments Peter & Tony.

Yes I suspected tar also but looking at it again today the black stuff looks like its been applied as a paint. It did get a little sticky last night, like a tar, when I was applying the hot steam to remove the wallpaper.

Anyway this morning I decided to remove the old Victorian fireplace and open up the original, so its gone back to the original brick which looks much better and opens up the space more. I saw no evidence of any soot or tar in the brick work, although some of the bricks in the centre of the flu looked a bit 'peppered', like they'd been under some chemical attack. I'll clean up the brick work tomorrow with a wire brush and it should make a nice original feature with the surrounding walls skimmed and painted.

This old fireplace will be sold for a song to someone who needs it more than me