Pre-heat the oven to 180C, 350F, Gas Mark 4. Par-boil the potatoes in boiling, salted water until just tender. Drain. Heat the oil in a frying pan and fry the onion for 2-3 minutes. Add the mushrooms, pepper and sweetcorn and cook for 2-3 minutes until tender. Stir in the herbs, spices and tomato pur?e. Meanwhile, melt the butter in a small pan, add the flour and cook for 1 minute. Gradually add the milk, stirring. Bring to the boil, stirring constantly until thickened. Add half of the cheese. Mix together the potatoes, vegetables and sauce and transfer to a casserole dish. Top with remaining cheese and bake for 30 minutes.