Squid ink can be difficult to find. It comes in small sachets which a good fishmonger can order.
To make the stock, melt the butter in a large saucepan and add the mushrooms, vegetables, bay leaf, peppercorns, thyme and garlic. Cover and cook gently for 10 minutes, stirring occasionally, until the vegetables are soft. Add the fish bones, squid ink, white wine and water. Bring to the boil, skim off any scum that forms and simmer for 40-50 minutes. Strain the stock through a fine-meshed siev