Pound the garlic and butter in a pestle and mortar until smooth. Season the garlic butter well with lots of pepper but just a little salt as the anchovy or bacon will act as a seasoning too.
Cut each potato into 8 slices. Coat the slices of each potato with the butter mix then reassemble each potato on a piece of double-thick foil. Cut the anchovies or bacon into strips and put between the slices. Do the same with the thyme sprigs and carefully wrap up the potatoes.
Lay the wrapped spuds on the cooler coals at the sides of the barbie for 1–1½ hours, depending on their size, turning every now and then, until tender. If you’ve got a gas barbie, put the potatoes right at the back over a low heat and give them plenty of time to cook. You might even want to give them half an hour’s headstart in the oven first.