Help - no leg of lamb, had to buy rolled shoulder. Also a roasties question.

[Northernlurker]

Lovely organic butcher said to roast it for 20 minutes at high temp then for couple of hours at low temp (it's 1.9KG) I plan to do so on a bed of roasted veg to lift it out of the fat a bit but am feeling nervous because I have people coming for lunch and I've never done shoulder before...

Also - if the ovens on low how can i do my roast potatoes? Can you slow roast potatoes for two hours with foil over them? And if so should I not par boil them?

i love roasting shoulder of lamb it is so tastey i would keep sheet of grease proof paper over lamb when cooking i cook mine for 4 hrs and the meat falls off the bone i turn heat up for last 40 mins for roast potatoes with foil placed over the lamb it doesnt or hasnt dried my meat out so far i cook roast shoulder alot

i do have veg chopped toms and wine in the tray with it