Heat your oven to 200°C/Fan180°C/Gas 6. Unwrap your lamb and pat dry with kitchen paper. Place it in a roasting tin and leave to come to room temperature.
Crush the juniper berries in a pestle and mortar. Finely chop the rosemary leaves and add to the mortar. Add 1 tbsp olive oil and a good pinch of salt and pepper.
Make a few small cuts all over the lamb leg with a sharp knife. Rub the juniper and rosemary marinade all over the lamb, pushing some into the cuts.
Roast the lamb in the oven for 40 mins. The lamb should be cooked medium after this time. If you would prefer it less pink, cook for a further 10-15 mins.
Remove the lamb from the tin and place on a plate, covered, to rest.
Pop the tin on a medium heat on the hob. Pour in 200ml of port, sloe gin, vegetable stock or water. Simmer for 15 mins, scraping any lamb bits from the base with a wooden spoon. Taste and season with salt and pepper. Strain the gravy through a sieve and skim off any fat. Serve the lamb with the gravy.