I was just watching the DVD of their German performances last night, including an early one of this line up. (Clive Bunker was my favorite of all their drummers.)
Ian has revealed, in interviews, why they settled on Jethro Tull. In the early days, with the original quartet, they went through a lot of names since they would play a club to less than critical success and, in order to get booked there again, they’d use a different name so the booking agents wouldn’t know it was the same band.
Jethro Tull was just the one that stuck when they were good enough to get a repeat booking.
Here’s their appearance from Beat Club in 1970. They tried to record another song, but Ian kept on getting muscle spasms from overwork. While they tried again, Beat Club overlayed their schedule for the previous 6 months where they only had 9 days off the entire time.