The Marans Club of Great Britain -
www.themaransclub.co.uk - is keen to get folk with the French varieties to show them at the various shows, with a view to getting them accepted by The Poultry Club. And there is a push to get those with stock/eggs for sale to let the club know so they can put buyers/sellers/swappers in touch with one another
If you follow the link it gives the Marans Club of France standard, which would be the one which would be adopted here.
Try to make sure you are breeding from a pure line, and not one which has been crossed in with other colours, which may then cause you problems further on down the line.
Its only £6 to join The Marans Club of Great Britain, and if you show your birds in a recognized class you can get a card and apply for a special Marans Club rosette, plus your contact details are available to other club members.