Thanks everyone for sharing your thoughts and recommendations. I took the advice of 'Chick' and 'Burbage' and contacted Southwark council pest control. They came swiftly and sorted out the problem for ?66.36. It was a five minute job, but it seems to have done the trick. We have our garden back and can open our windows again, hurrah! Thanks 'Take Note', 'Dulwich Fox', Sue and 'SpringTime' for sharing your experiences. I think calling in the professionals was the right thing to do in this case.