Preheat oven to 325F/160C.
Make incisions into the fatty side of the lamb joint with a small sharp knife, evenly spaced over the top and side of the joint around 1-2in apart (3-5cm).
Mix together the honey, lemon, mustard and oil, pour it over the lamb then rub all over.
Peel the garlic and cut into long strips, around 6-8 per clove. Break the rosemary stem(s) in to approx 1in/2.5cm lengths (easiest to break the stem just above where the leaves sprout out so you get a little 'point') then stuff a piece of rosemary and a piece of garlic into each incision in the lamb.
Sprinkle a little salt over the lamb and roast for approx 20min per pound/450g for medium-rare (around 25min for medium, 15 for rare), although time may vary with your oven, size of leg and if boneless - for me, this time gave rare rather than medium rare. You can check with a thermometer (around 125F for rare, 130F for medium-rare, 135-140F for medium, 155-165F for well done).
After around 20-25 minutes, add a little water to the bottom of the dish (a couple of tablespoons) and repeat later if a larger leg joint.
Remove from the oven and allow to stand for around 10-15minutes before carving. Scrape any stickiness from the cooking dish and mix with the juice/water that should be left from cooking and pour over as a gravy. If you prefer thicker, you can add a little cornstarch/cornflour.