Take the leg of lamb out of the fridge 1 hour in advance of cooking. Preheat the oven to Gas mark 4, 180°C (350°F).
Mix the garlic with the oil and rosemary. Use your hands to coat the lamb all over with this mixture, rubbing well in. Season.
Place the lamb on a rack in a roasting tin. Pour the water and the wine into the tin and roast for your calculated cooking time.
Medium: 25 minutes per 500g plus 25 minutes
Well done: 30 minutes per 500g plus 30 minutes.
Mix the honey and mustard. Ten minutes before the end of cooking time brush the mixture all over the lamb.
When the lamb is cooked remove it from the oven, wrap the lamb in tinfoil and allow to rest for 10 minutes. Meanwhile, place the roasting tin over a medium heat and add 150 ml of boiling water and the redcurrant jelly. Stir well with a small whisk or spoon to release any rich sediment. Taste and season.
Strain the sauce into a jug. Carve the lamb into slices. Serve with the sauce, roast potatoes and a salad of chopped cherry tomatoes, sliced red onion and basil.