While other companies, such as Boots, saw the problem coming and shifted their pension funds from equities towards bonds early in the stock market slump, BA waited until near the bottom of the market. Had they recognised they had missed the boat, and waited, the NAPS fund would have recovered along with the stock market. It's hardly the employees' fault.
With BA profits rising and fuel prices falling it's becoming harder for BA to justify the claim they can only afford £500m to fix a problem largely of their own making. Things are slowly beginning to turn in the unions' favour. I see no reason why they should rush into a settlement they may regret.
On the other hand a commitment by BA to continue the final salary scheme is worth an awful lot of money to its employees. Ask any pensioner.