Selling spare seats in the mid week at a deep discount can be useful for an alrlines cashflow. Silverjet have gone past this point and it seems they are doing more of a fire sale. In the January sale almost any flight on any day was discounted. When people who would have paid the full price can buy a discounted ticket then income goes down.
Silverjet would need something like 75% load factor at an average return fare around £1099 (inc tax) to cover the flights costs. If 20% of the seats are sold at half price you would need over 85% loadfactor to cover the flights costs. Silverjet need full price ticket sales to keep going and it seems they are not getting them.