Before undertaking such a task as insulating your roof space or loft you have to ask yourself. Do you like spiders? LARGE spiders? Finding said large spiders on your shoulder? Because if you don't you should run away now. Also unless if you are equally partial to having itchy fibers making their way down your trousers to your bum this may not be a job you wish to do either. That said if you pay someone to do the job you lose the opportunity to inspect closely all the areas of your roof space such as the condition of the woodwork, roofing felt, masonry, the spiders and the odd wasps nest.
In our loft we currently have no flooring laid down, this is another job on the list, however this made it easier to see what I was working with. A word of warning, if your loft has no floor - mind you don't go stepping between the joists as you'll end up in the room below... probably with a spider on your shoulder.. and an itchy bum
Between our joists, in the void, was a couple of inches of rubble and dust (presumably from when the roof was redone in the 80's) the old insulation was laid on-top of this which in turn was caked in dust and had flattened down quite a bit, providing a limited thermal barrier. The first job was to remove all of the accumulated rubble and set about laying down insulation. The sheer amount of dust meant that it'd probably kill the digital camera, so apologies for the lack of photos, there is also no lighting up there so I made do with a couple of angle poise lamps.
These rolls are available at around £12-16 each and should make a substantial improvement. I think it's common for people to use this material when insulating the insides of caravans, in-fact I've seen it used on refurbished aluminum Airstream caravans which are probably freezing without adequate insulation.
So now the task is complete and already there is an improvement, the house has gone from feeling like being sat on the outside of an igloo to being at least sat inside of one, especially in our bedroom. This might help once we sort all the various drafts coming from everywhere else in the house so the next job should be new windows but these are hugely expensive and we'll have to save for new units, for now I have taped up the draftiest ones. A bonus point was that whilst up in the loft I noticed that our house does have a wall cavity and hopefully this will mean we can get some insulation pumped into the void so I'm going to see if I can get a grant to do this.