wrote:
What's to stop me having a book of Saver tickets and, when the
Inspector asks just hand one over?

The rules. http://www.tfl.gov.uk/buses/ini_207.asp
says:

"Saver ticket holders must board through the front door and present
their ticket to the driver as usual"

Is he really going to fine me
because I entered through the rear doors because of the crush at the
front of the bus, and my inability to get to the driver (certainly
likely to be the case on a rush-hour 521) because of the crush?

I don't know. But they would be entitled to charge a penalty fare.

PaulO