Please can anyone identify this insect?

This insect flew into my conservatory yesterday afternoon. The body is about 6cm long and 1.5 cm wide. The wing span is approximately 15 cm, so quite a large insect. My initial though is that it might be a dragonfly! but I think this is too big to be a dragonfly. Would be great to know what this is. In the end I had to use my son's fishing net to guide it outside.