Thank you all for your replies.

Jochen: I looked up "Kremer Pigmente" they appear to have a very useful selection of chemicals. what they are selling as "Liver Of Sulfur" is listed as sodium polysulfide while this would be useful for Gerald's recipe it looks like it would be a pain to get it shipped as they ask for ID card details (thankfully we don't have ID cards in the UK) and won't provide an online quote for shipping to the UK. however I will keep them in my bookmarks for future reference.

Gerald: Thanks for the recipe. I am not a chemist, your recipe calls for 25ml of Sodium polysulfide, 30% soln. The instructions I have to make sodium polysulfide are:
Make a solution of 5 g of sodium hydroxide in 100 cm3 of water. Heat the sodium hydroxide solution until it is boiling. Add the 8 g of sulphur and a drop of washing up liquid to help the sulphur mix with the aqueous solution. Continue to boil the mixture and stir until the sulphur has dissolved to give a dark red-brown solution of sodium polysulphide. Decant the liquid off any small lumps of sulphur which remain undissolved.
Does this give me a 100% solution of sodium polysulfide? which I can then mix 7.5 ml sodium polysulfide solution to 17.5ml water to make 25ml of 30% solution.

I have ordered 100g of potassium sulphide from thrivechem as it is described as Potassium sulphide (poly) which Rose Chemicals describe as potassium polysulphide. It's all a bit confusing, I will let you all know what turns up an if it is of any use.