Latecomer to this thread, but I think things seem to be a little confused in economic terms. The elasticity of demand clearly does depend on the opposition. Having different categories of games already recognizes this! - Cat A = inelastic, high prices as demand is high regardless of price Cat C = elastic, lower prices to stimulate demand.
If the stadium is not full, then someone is not doing their job as well as they should, as empty seats always = loss of revenue. Not only from the ticket cost, but from additional spend at the stadium.
So reducing the cost of the empty seats at Cat C games is one possible solution. Another would be last minute, at the ground sales of the unsold tickets at a discount of £5-£10. That seems a lot, but remember, if they otherwise stay unsold, you make £0. Also, you get people to the stadium, where you could potentially sell merchandise, or one thing that I don't know why no clubs have done - possibly broadcasting rules or police / safety regs? - Have a cheap standing area with a giant screen for 'overflow' fans.