[ainarssems]

-16C is probably low enough for England, but is it 25/75 or almost neat antifreeze 95/5 as both would be around -16C?

I would guess it's more likely to be 25/75 but it's possible that somebody filled it up with pure concentrate thinking it's ready mixed. Another thing to keep in mind that it's not just freezing temperature that you need to think about. Antifreeze also contains corrosion inhibitors, conditioners for rubber like hoses and seals and likely other additives, antioxidants to prolong antifreeze life. Normally you would use about 40-50% antifreeze and rest water. If you have smaller proportion it could affect other properties and reduce life.