C, the explosion was heard 24 secs after the flash in the sky. That's exactly 27,000

[Sir C]

Yes. Mach 1 = 343m/s

x24secs = 8,232m (Roughly five miles.)

=27,007 ft.

Where is my maths wrong? What have I missed?

The aircraft is not directly overhead the photographer, is it? It is, apparently, 5 miles away in slant distance, so perhaps 4.5 miles away horizontally. Remember your right-angled triangle calculations?