cheap antifreeze

I would go to a car spares place, supermarket would be next best, halfords a last resort. But why would you want cheap anti freeze? Good stuff such as genuine Vauxhall A/F is only about 7 quid a gallon.

I wouldn't use any cheap antifreeze. Antifreeze is a complicated product, not just water with glycerin. Antifreeze performs three important functions:

1. Cooling the engine
2. Preventing frost damage
3. Inhibits corrosion of the radiator and cooling galleries

In particular 3. is most important. Various products have different corrosion inhibition life times. Cost is small if lifetime is 3 or 5 years.

Personally I'd pay a few quid more for the long life stuff, knowing it was good for at least half a decade.

I'd buy OEM stuff from the dealer or from GSF/Euro. It's not like it's that expensive, and using the wrong stuff or mixing incompatible ones is a load of hassle.