I was there (on a trip from the States) at the first performance at St. Paul's and the reverb at the end of the first movement was breathtaking, you had to be there!
The next night my wife and I had dinner on the South Bank and I just "happened" to have us stroll across the Millenium Bridge. "Whadda know, there's St. Paul's!", I said. I asked my wife, "Aren't they doing the Mahler 8th again there tonight?". She said, "You know damn well they're doing the Mahler again tonight, only a Mahler nut like you would want to hear it again." At that time of the evening, I knew the symphony was concluding, I looked around to see if there were any entrances that weren't guarded, but alas, I couldn't "wander" in!