I reckon that the "correct" day for December salaries to be in accounts is 27th. ie leaving 28th and 31st as the two clear days. As the company appears to be almost bankrupt, then I for one will not be working in January if I am not paid for December. Remember we are paid in arrears. This is not strike action. If NATS breaks the contract by not paying its debts then it is hardly strike action not to go to work.
I would also suggest that if salaries are not paid then NI contributions would not have been paid which begs the question "Are you insured at work?"
Z