Hi, AP.
I don't know how to provide the link but if you google 'Myland's' or 'John Myland'
they are an old established paint and varnish company in south east London specialising in all types of finish. They will undoubtedly have what you are looking for.
As a bit of an amateur woodworker myself I would recommend a water based dye, often provided in concentrated crystal form which you dilute in water. If you start off with a weak solution you can always go over it again with a stronger dose. Or, simply buy a dark stain. (see their catalogue). Once you have the colour of your choice, apply about three coats of polyutherane varnish. I would recommend thinning the first two coats 50/50 white spirit to varnish, rubbing down lightly between coats, then a full final coat.
I actually went to trade school with John Myland in the 50s. A great firm.
Good luck.
D