Thus, a 600g lamb shoulder will be done in about 65 minutes. For slow roasting, preheat the oven at 180C/350F/Gas mark 4, then allow 35 minutes per 450g/1lb plus an extra 35 minutes. A 600g lamb shoulder will be done in approximately 80 minutes.Find a delicious roast lamb recipe at Delicious Magazine.or slow roasting, preheat the oven at 180C/350F/Gas mark 4, then allow 35 minutes per 450g/1lb plus an extra 35 minutes. A 600g lamb shoulder will be done in approximately 80 minutes. Find a delicious roast lamb recipe at Delicious Magazine.