Here is a link from the BBC going back to 2004. It is comical because it states that they were going to test the flood warning siren but it was raining too heavily to do so!
http://news.bbc.co.uk/2/hi/uk_news/e...re/3557654.stm
Considering that you guys are near to the coast down in Dorset, would bet money that it was a flood warning test that was heard.
Here is a link to West Dorset District Council who carried out a study, mentions flooding possibilities
http://74.125.77.132/search?q=cache:...lnk&cd=2&gl=uk