Do it yourself. Just get a load fibreglass, and if you've got double-thickness walls, then it's easy, just stuff it down between 'em. If you haven't, it's a bit more difficult - but not much more, you're just going to need to nail it to the wall - and then whack a layer of plasterboard on top. If you called blokes in vans out to do it then that's all they'd do, so you might aswell save yourself the money. Don't fall into the temptation to compress it loads so you can stuff more in, 'cos then it doesn't trap as much air in it and isn't as insulative.