UKC

All round sleeping bags ie inc winter for camping and potentially bivying. Currently looking at:  

rab ascent pro 800, 
rab ascent 900 and 
me helium gt 800. 

main considerations are weight, pack size, warmth and water resistance. Also important that a long version is offered. All similar specs but some slightly vary eg quantum vs quantum pro and 15°C vs 18°C. Also the fact Rab uses rab sleep limit vs ME using standard rating. N.B I understand a good sleep mat is equally important but not looking for advice re this yet. 
Any of your own experiences or alternative ideas much welcome : ) 

Does it have to be a water resistant sleeping bag, or are you going to use a bivvy bag with it?

I have a Alpkit Alpinedream 800 which I bought cheap on a deal last year. Its in the same rrp price range as the mentioned options but you do get higher fill power down which apparently makes a difference (who really knows...). All I know is that I was warm ish on a very exposed bivvy with temps below -20. Bivvy bag or small bivvy tent always worth it imo, I have the cheap alpkit bivvy bag and it's fine. I also got the Lomo silver foam fold up mat, which is a knock off of a thermarest product and that was fine as well. I was pretty chuffed to be warm in those conditions with a reasonably lightweight and, more importantly, affordable set up.

Water resistant would be preferable, in any case i dont plan on sleeping in just a bag in rain haha but, or at least hearing peoples own experiences with regard to down and condensation from inside tent and or bivy bag. 

I recently got a mountain hardwear lamina 0 degree F and it fits all criteria. Have a look because I think they are harder to come by in the UK now but definitely worth it for the price. Haven't had a sleeping bag both this light and warm for around 100 quid

ONLY those considerations then?  I think you covered them all 😅

To be honest, both ME and Rab make great bags.  All mine are ME but that's personal preference.  I wouldn't get too excited about hydrophobic down, so rather look at water resistant shells.

Considered the ME Glacier?  Not as light/packable as the Helium, but it's got a better shell (more water resistant) and a more generous fit.  I have a glacier 750 and it's done me well for high altitude and winter bivis/camping.

I've got an ME Glacier 1000 and it's the absolute DB's..

Barbeg 

Mountain Hardware Ghost SL.  You can't get cold in this!  Roomy enough to dress/undress inside which is essential in my opinion for a winter bag .

Very pricey though......even second hand . I'm moving to warmer climes so I'll be selling mine at some stage, but it is probably more bag than you need.

I asked a very similar question here last summer.  After listening to the forum and a bit of research on Google, I bought a Snugpack Softie Expansion 3 Light.  Great sleeping bag.  I took it up to the Cairngorms in February this year.  FYI I am 6ft 4" and I fit fine. Not much to spare though.

https://www.absolute-snow.co.uk/snugpak-softie-expansion-3-lightweight-slee...