How do they actually work, i.e. lacing up the back and mats etc?
Most have some type of strap set-up underneath that you can tie under your mat ... I very rarely bother to use them unless it's VERY cold.
Could i use one with my 3/4 length ccf mat?
Yes ... most quilts still have a footbox like a sleeping bag.
Does anyone have any suggestions about a good, budget (preferably around £150) and light (around 600g) quilt that i could use most of the year round?
There's not much choice in the UK but a bit more in the US. My first port of call would be Enlightened Equipment in the US, Flatfisy says they've got a sale on at present. Their stuff's very well thought out and well made, they'll also do custom stuff too. The Revelation X was / is their budget model and is superb:
http://www.enlightenedequipment.com/
Thermarest have had a dabble with quilts, the Navis and the Alpine (they call them blankets):
http://www.ultralightoutdoorgear.co.uk/ ... g-bags-c88
Another entry into the quilt market this year were Cumulus. They do 4 different weights (150 - 450) and I'd suggest the 350 would suffice for 3 season use if you wear your jacket:
http://www.highmobilitygear.com/PBCPPla ... ID=1172579
There's a few others out there but they're going to start going waaay over budget ... if it were my money I'd buy an Enlightened.