Hi Ed and welcome
Sleeping bags.... where to start?
2 main types - down fill or synthetic.
Down will loft more and be warmer weight for weight, but get it wet and it's useless. Tends to be more expensive as well unless you go for surplus - British Army '58 pattern are fine if a bit bulky.
Synthetic doesn't loft as much as down but maintains some of it's insulative properties when wet.
Ideally you don't want to get any bag wet as performance will deteriorate whatever it's made of.
You don't mention where you'll be using the 'bag so I'll assume it's just UK use.
Personally I have 2 - a down bag I got from Karrimor 10 years+ ago. It's rated to -30 which sounds extreme but everyone sleeps at different temperatures and I've used it October to March without being uncomfortable. It packs doen well and lofts like you wouldn't believe.
For the rest of the year I have a Snugpak Softie 3 (I think)

Used to be the world's smallest / lightest bag, packs up to nothing and weighs 2 lbs. I can increase it's temp range with a thermalon liner, again which packs up small.
Have a think when you're going to use it most, over rate the bag rather than under i.e get something too warm - it's far easier to cool down than heat up.
Finally, when you get to specific models, ask again on here as people seem to have a massive variety of everything (me included, just don't tell the wife!)
Hope that helps, have fun with the forum.
Chris