Hard situation, and I have sympathy for you. Get your flatmate to pay you back weekly, say £10 a week or get her ex to pay up.
My flat mates friends used to come stay for weeks at a time and he said they'll buy food, rent movies, pay their share of the phone bills and help out with elec/gas... problem was I worked long hours and only got home at 1:30am if I didn't miss the last train or it hadn't broke down so all the food had gone and they were usually fast a sleep in the living room so I couldn't watch the movies they rented, my weekly 'rent' which covered gas/elec/council tax and rent never changed either