Pre heat the oven to 180c, in a large pan heat the oil, then add the onion and garlic and sautee for 2-3 mins and add the bell pepper sautee for a further 2-3 mins. Add the sausages to the pan and brown on all sides.
In a jug mix together the hot stock, beer, tomato puree and marmite (stock will have to be hot to melt the marmite). Add the stock mixture to the pan along with the tinned tomatoes and simmer for 15 mins.
Transfer the stew to a casserole dish and cook int the oven,for 30-40 mins, untill gravy has thickened.