It's good to see Prof. Heald's "Choir Directors" paper on-line. I hope it stays up for a while.
My understanding of how record deals work is: the recording artists (not the songwriters) get the worst part of the deal, because they often have to pay promotion costs from their share--though songwriters are capable of signing bad contracts, too. The record label is in the strongest bargaining position and (again according to what I've been able to find out) get's the biggest share of the proceeds from sales of the record.
The songwriter in theory has an additional source of income besides mechanical royalties from the record: performance license fees from radio stations that play the record. In practice, the performance licensing system has its own problems.