In a pestle and mortar crush the garlic, rosemary and anchovies together with the salt and pepper. Once you have a chunky paste mix this into the softened butter.
Spread the leg of lamb generously with the softened butter mix and place in a roasting dish.
Roast in the oven for 15 minutes; lower the oven temperature to 180C/160C fan/gas mark 4 and roast for 1 hour more, basting from time to time with the juices.
Meanwhile, in a hot frying pan add the cold butter so its sizzling slightly and then add the white wine, the radishes and peas just to warm through and emulsify. Add in the mint last minute.
Then fifteen minutes before the lambs cooking time is complete remove from the oven, pour over the radish and pea mixture and then return to the oven for the final 20 minutes.
Once cooked remove from the oven, cover in foil and leave to rest for 10-15 minutes before serving
