fuel lines going into the tank, need to drop the tank to get to the fuel pump. not worth the risk using a dremel, would be first time using one outside a controlled environment too (lab)
If the worm drive is seized due to corrosion can't you spray it with penetrating oil or Halford's Shock & Unlock then put a small socket (better grip than a screwdriver) on it and wriggle the worm drive to-and-fro?
Just get some pliers on the clip and twist the actual pipe around until you can get access to the screw. Or undo the pipe next to it that's blocking your access.
At the end of the day, someone did the clip up, so the screw will be accessible from somewhere.