They can close the scheme to new members any time they want. In regard to asking for a contribution of 5%, normally employee's/unions would be consulted. It is a change to your terms and conditions of employment. The question you have to ask yourself is do you really want to fight it because making a 5% contribution to your defined benefit pension is still very cheap for the type of benefits you will receive and given the times that are in it, it may be better to just accept it.