If it hasn’t you want to fit a 1st fit product, if the windows are just being replaced you want to select a 2nd fix cavity closer.
– How large is the cavity?
Simple, if the cavity width (the distance between the brick and block) is 70mm then you require a 70mm wide cavity closer.
– Do you want to comply with Building Regulations C, B and L?
If you do then select an insulated fire rated cavity closer with a minimum thermal resistance path of 0.45m2k/W, this value is also known as the R Value. All cavity closer manufacturers will be able to provide you with this information.