How reliable are the EN ratings?

The sleeping temp range I listed at the start was the extremes High and Low 30°C (85°F) - 0°C (30°F).
It will be more like 25°C (75°F) - 6°C (43°F) as the high and low.

I'm thinking about these two
Sea To Summit Trek I Series Down Sleeping Bag
Sea To Summit Trek 2 Series Down Hooded Sleeping Bag

I'm also thinking of using this, will it be worth using the liner to keep the bag clean?
http://www.wildearth.com.au/buy/sea-...ine/asilkcsyha