Most Foolproof method is get on eBay or gumtree and pick up a second hand sous vide machine for about 30-40 quid
Stick it in a water bath at 52 (rare) 55 (med) for 4 hours then give it a pat dry, salt,oil pepper put it in a preheated oven at max temp for 20mins
Rest for 10min
Second best is get a meat thermometer for 20 quid off Amazon the type on a long cable so you put the unit outside the oven
covered in a low (140c) oven until internal temp is as above then out while the oven heats to max and same as above
Final is follow any pink leg of lamb recipe
Half an hour hot oven then med oven for 15min/lb or so then a good 20min rest