Haven't come across it before, but things change all the time , and these are very difficult times for any company ... least of an insurance company that appear to be based 280 miles away from Manchester, in Bexhill on Sea !
I expect the detective agency involved just want your NI number to confirm that it's really you that they're speaking to/or corresponding with ... they can no doubt check your NI number or they wouldn't bother asking for it .... to be honest it doesn't sound dodgy to me , and Hastings seem to have immediately levelled with you regarding their use of thiis detective agency .... i don't think you've got too much to worry about.
If the text from the detective agency was a fraudulent one it would be one hell of a coincidence thatt they would even know where to send it , or who to send it too .... if they weren't liasing with Hastings!
From what i can see nobody involved in your case has given you any real cause for alarm (other than the towrags who nicked and torched your car of course!) , so just follow procedure ... if you DON'T follow it you'll soon find out whether or not they're genuine or not because your claim just won't go anywhere!