Hamelin Synthetic Sleeping Bag

Have a similar one from Mammut (same temp range). At 0 C you should add some long undies or a liner. Have a warm soup or drink before you sleep and you're A-OK. Also, it depends on how you camp. A tent usually raises the temperature a bit. So if outside is - 1 C, the inside might be 5/6 C. All depends on circumstances. Sea to summit is a solid choice though! Price seems okay.

If the temperature can dip below 0C, it is barely ok. But with a thin and lightweight liner, it will be more then enough. The weight of the bag is also pretty nice at 1,263 grams (2 lb 13 oz). I personnally think it's on the expensive side, but Sea to Summit is a top quality brand.

S2S do have their bags EN tested so that temperature rating is accurate, with the caveat being EN testing is done with the dummy wearing winter base layers, socks, gloves and a beanie. It is a skinny bag and not suited to larger people tho, I have the S2S "BaseCamp" the larger version of this bag and I use it for car camping and find the modelling accurate for me as an older camper who likes comfort

A good synthetic sleeping bag would weigh less than 1.5kg (say, Sea-to-Summit Hamelin), will allow quite a bit of movement inside, can be turned into a blanket when unzipped.

The last bags we had were high end sea to summit that were rated to 0 to minus 15C. They were rubbish. We bought Neve gear quilts. Very light, compact and the best things ever.  Much easier to get in and out of. We still use a liner/sheet sac. If you can afford to get them made, as opposed to off the rack,  do that.   Don't ever use your bags outside, especially next to a fire.  Thats only for instagram pics. Pick a bright colour for the inside and a dark outside colour. We use trekology blow up mats.  Exped deteriorate too easy.  We use our gear a lot. Usually go for 1-4 months and sleep out 90% of the time. 

I have a disco 30, M44. I went camping in May (65°F low) and September (60°F low) and was too hot, had to unzip and use like a blanket. I also went further North in October (45°F low) and was comfortable. My daughter has a Sea to Summit (30°F) and she was cold. Liners are an option but a royal PITA if you have to pee at night. In hindsight, I'd have gotten a lightweight quilt for warmer months and a 15°F for her.

I have a sea to summit backpacking bag that’s 15° rated and once it’s in a stuff sack it’s incredibly small for a 15°. Coleman is basically bottom of the barrel, sleepover style, when it comes to sleeping bags, I’m surprised you aren’t freezing to death at night.