Pete.. there is a UK based sleeping bag manufacturer which makes high quality bags which weigh half a kilo and crush down to the size of a baseball. ..
To get the idea, look at this Australian equivalent [ scroll down to the Palm 1 and Palm 3 bags ] to get an idea of how small you can pack the high technology bags.
http://www.extremeequipment.com.au/sleeping1.htmlThe UK equivalent is on issue to the British Army. I guess you need to consider expected climatic conditions and whether you are a 'hot' or 'cold' sleeper.
My experience is that you can easily add more thermal wear and take a 'lighter' bag, if size and weight is a problem...
Hope this is helpful.. I cannot remember the UK manufacturer's name, but they have advertised in outdoors magazines here in Australia, so should be easily found in UK outlets...
PeterR