Season the lamb evenly with a few pinches of salt and 2 tbsp sunflower oil. Brush the rest of the sunflower oil round an oven tray pan and spread the lamb round in a single layer. Place the tray in an oven preheated to 210C and roast for 20- 30 mins pouring off the fatty liquid and turning half way through. The meat should be nicely browned. Add all the vegetables and the browned meat into the slow cooker. Thoroughly mix the cornflour with a bit of cold water and add it and the BBQ sauce (rinse the bottle with the 50ml water and add) to the slow cooker. Add a few generous pinches of salt and gently mix together.
Set the cooker to "high" and cook unopened for 5 hours.
Serve with plain rice, creamy mash potato, cous cous or warm bread.