I do lamb shoulder very much the same, i put a piece of baking paper over lamb to stop alfoil sticking and put in 180 degree oven for 30 minutes, i then drop the temp to 120 and leave for 4-5 hours, i then hit the temp up to 180 and remove foil and the baking paper on top of should and let it brown, lamb shoulder done like this is so good, i cant understand how people eat lamb that is still pink or bloody inside..
