Thanks Ruud. I'll give the extra mat a try and I can get a fleece liner for £15, maybe try that. You don't think cutting my losses and getting a higher rated bag would be a better option? (My current bag weighs 1.95kg, a liner would be 0.85kg, equalling a total of 2.8, the elite 5 next bag up from mine only weighs 2.4)My two cents:
I use an inflatable Thermarest which I use in conjunction with a cheap Z-style Karrimor foam mat. The Karrimor foam mat can be used to sit on, be used as a kneepad in a canoe and so on and doesn't add any noticeable weight to your pack actually. It is especially great to lay on during breaks on longer hikes. It adds a tremendous amount of comfort for its price and weight. In summer I lay my Thermarest inflatable on top of the foam mat to keep it safer from needles or other sharp ends. In winter I lay the foam mat on top of the Thermarest, as it is good practice to lay on the warmest (highest R-rated) mat you have with you.
Ever since I use a Sea To Summit Xtreme Reactor liner I've slept so much better. Less chill in the mornings, a second layer to pop over your skull... The fabric is pretty stretchy, I don't feel like I'm getting tangled in it. You might want to try a cheaper version to test out wether it works for you or not (the entanglement).
I don't think you need to make any changes in regards of your bivybag.