Your haggis is already cooked and only needs thorough re-heating. Wrap the haggis tightly in tin foil and place in a large saucepan of cold water. Put on the heat and simmer gently for around 1 hour (45 mins per lb). Do not allow it to boil or this could burst the skin. Do not let it boil dry; keep a kettle of boiled water on hand for top-up if required.
While the haggis cooks, prep for your sauce. After haggis has been heating for 30 mins, check if it needs a water top-up, and then begin your sauce.
Add the butter to a frying pan and melt over a low-medium heat. Fry the chopped shallot until softened. Add the whisky to the pan and carefully light with a match or lighter. Stand back! The initial flame can be aggressive but will burn out. This makes the sauce less bitter, but if you are not comfortable lighting it then you can just allow the whisky to cook off for a few minutes and reduce a little to evaporate some of the alcohol.
Once the alcohol has burned off, add the stock and simmer for 5 mins to reduce by half. Turn the heat down low and add the cream and mustard, incorporating well with a whisk. Heat gently for 5 mins to thicken slightly. There will be no need to season with salt as there should be plenty in the stock, but you can add pepper, to taste, at this stage. Remove from heat and transfer to a warm jug.
Remove haggis at the end of cooking time. Probe centre with a meat thermometer to ensure it has reached 74°c, remove from foil, drain excess water, and serve with your whisky sauce, and clapshot or mashed tatties.