I’ve had and got quite a few sleeping bags. Currently have bags/quilt from Snugpack, PHD, Cumulus and Rab with temp ratings from 10C to -30C.
Are you a warm or cold sleeper? What temp range are you expecting to use the bag in?
If you want lightweight then look at PHD (pricey) or Cumulus (sold in the UK as Criterion). Alpkit are fine but use lower fill power down than PHD and Cumulus so the bags are bulkier for a given temp rating, this is reflected in the price. If you are wanting to use the bag into autumn then consider a bag along with either a quilt or overbag to increase the temp rating when you need it. Unless you are continually camping/bivvying in a monsoon then modern treated down will stand up to minor dampness humidity. Synthetic fillings have improved massively in recent years but they still don’t have the lifetime of a quality down bag.
OK, can of worms duly opened!