Very often the police seem to run training flights over the area, usually late at night, and lasts for around 20 mins. It's been a quiet year this year. Usually happens about 5-10 times across the summer months. I presume they are testing infrared search equipment given the time of day. It is very annoying, but however annoyed you are, don't be tempted to even shine a torch at them in frustration, you can be prosecuted for "endangering an aircraft". Yes I am serious.
Robert Morley ● 1496d