i forgot 2 mention u can use fernox inhibitor in ure system,it is added to help reduce the corrosion process in central heating system,
Could you use Bluecol car antifreeze in the radiator system? as this stops corrosion in a car radiator system, which is basically the same thing and would also protect against frost
Not to mention that most car antifreeze products recommend a 50/50 mix, so it'd cost a small fortune to do the entire system. Of course, you wouldn't need its antifreeze abilities, so you may be able to use less, but then the inhibitor probably wont be anywhere as concentrated as X100 or similar.