Preheat the oven to 180°C, 160°C fan, gas 4. Put the sausages into a roasting tin and cook for 30-35 minutes or until cooked through.
Meanwhile, steam the potatoes in a large steamer for about 7 minutes, then add the carrots and leek. After a further 5 minutes, add the cabbage and cook for 2-3 minutes. Season with pepper, then roughly mash all the veg together in a bowl.
Heat half the oil and butter in a frying pan and add the bubble-and-squeak mixture. Pack down to make a cake. Fry for 5 minutes, then place a plate on top and invert the cake onto the plate. Heat the remaining oil and butter, then carefully slide the cake back into the frying pan, brown side uppermost, and cook for a further 5 minutes.
Cut into wedges and serve with the sausages.
Cook's tip: if you fancy onion gravy with this dish, add 2 heaped tbsp onion marmalade, a little mustard and some beef stock to the sausages for the final 15 minutes of cooking.