Ok so I think I'm over thinking this...
I have a 1kg shoulder of lamb that I intend to cook on a low heat for about three hours. Would that work?
Also then what happens with cooking the roast potatoes, because I would need to up the temperature in the oven?
Or would I wait until the lamb is cook and keep the lamb warm in the slow cooker or something?
Oh should I just cook the lamb on 190 for an hour and ten as per the packet?
My brain is just melting!
Thank you! 
