Just to report on this, had my second service done today (car is two years old - down on the sheet as '18,000 mile service' - it's done 8,000! Guess it goes by the age) - ouch, £380...
I clean forgot I had removed the exhaust spring. No mention, no comments, exhaust system is apparently fine. So it stays that way! I have noticed no difference whatsoever in performance without the exhaust flap spring, it just pops more. So that's a win, then.