If you look at the places I have come across this particular species of crane fly, which I believe to be Tipula lunata, you may notice that the sites are predominantly woodland areas; in fact damp woodland and that is the preferred habitat for this particular species.
Crane fly identification from photographs is not ideal, the experts prefer a specimen to examine for minute details, but for the casual naturalist like me I am happy to adopt a more 'percentage' based approach. This is how I work; firstly I form an opinion on the type of insect I have in front of me, in this case it is obviously a crane fly, its size and those outstretched wings could hardly be anything else. I then think about the date and narrow down the options by using my field guide or other reference source to home in on those species likely to be about at that time. Linked to this I will look for species found in Southern England.
With a much shorter list of possibles now I turn to the habitat and look for species preferring that habitat type, in this case damp woodland. By now the options are much fewer and I can look for identifying features and here it is the pattern on the insect's abdomen and the presence of two dark patches, one on the front edge of each wing. Finally I look to see how common the species is, if it is rare then possibly I have made a mistake as statistically I am more likely to to see common species.
So I may possibly be wrong but I am probably right with my conclusion but, when all said and done, I am not involved in scientific research here, just trying to provide a guide to the nature of Dorset for other amateurs and casual observers like me.