For UK 3 season use, you're going to be hard pushed to find a synthetic bag with a comfort rating below 5°C, that is less than about 1.2KG.
Example:
http://www.montane.co.uk/range/sleeping-bags/prism-sleeping-bag £160, approx
Yeah I looked at the Prism one, and it is cheaper than the OMM 1.6 too. If I really struggle with the OMM I will take the financial hit, sell it on and get one of those.
I have plans for a bit of an adventure this side of winter now, hence bringing the bag purchase forward.
The OMM1.6 is no doubt a gamble. It's not rated or sold for a 'comfortable nights sleep' in October. But, the more I thought about it I'll not be using it for 'proper sleep' purpose outside of a warm summer night. It'll be emergency hunker down and sleep for an hour or 2 in the middle of an unsupported 600 in the spring/autumn. There will be no chance of me overnighting in the winter and if I even considered that level of harshness I know it would be with much better rated kit, and on a relatively flat route as the bike would be so laden!
This will be used on exceptionally hilly rides where weight actually is important for me. I'm not big myself so extra weight is very noticeable. I am hoping to carry this little OMM, and the Thermarest matt and a bivvy in an alpkit bar drybag/koala saddle bag combo with my Paramo Coat and normal kit and, for now, not run panniers. The bag is significantly lighter and smaller than anything else (Primaloft based) out there.
Which brings me on the the coat - I'll have a decent warm Paramo coat to sleep in, dry base layer and dry socks in the kit and the Thermarest which will all increase the warmth a fair bit and that I would be carrying anyway.
I have also been pondering this wee blanket which has been getting great reviews and will tuck in nicely in a corner of the kit bag
http://www.backpackinglight.co.uk/practical/XA107.html Plus synthetic does have some advantages over down - works when wet. Doesn't need as much looking after. Both of which are important for me.
I will be testing it over the next month and will provide feedback