Remove the skin and bones from the Arbroath smokies and break the flesh into large flakes. Mix the garlic with 40 g of the butter and spread it over the base and sides of a 2.25 litre shallow ovenproof dish.
Layer the potatoes and flaked smokies in the dish, seasoning each layer lightly with salt and pepper. Finish with a neat layer of potatoes.
Mix the cream with the milk, pour it over the dish and dot the top with the remaining butter.
Bake for 10 minutes, then reduce the oven temperature to 180 C, 160 Fan, Gas Mark 4. Continue to cook for an hour until the potatoes are tender and the top is golden brown.