B&Q. Except its called Aquaseal. comes in rolls, not sheets. dont buy the LARGEST rolls, but the size down from it. thats got the best price per square inch. You'll need 2 rolls to do the boot, but if you are hardcore you can use up to ten. i think my car has about 5, and thats plenty. boot floor and sides, bootlid/hatch, rear quarters, the doors, and a couple of little bits under the dash all have at least 1 layer. sounds really solid when you tap on it, and hardly any sound escapes the car. much cheaper than adding another amp and 2 sets of speakers, but more time consuming.
Buzz
The champion of the Bravo 1.4, and owner of a Bravo 1.8 Member Page
I'm sure buzz has done his roof, his cars weighed down by the stuff! I'm sure i saw a post yonks ago about him adding some around the insides of the door mirrors and sunroof, the man is dedicated and loves his bass
I love my bass also but my whole car just vibrates!
I bought some of the aquaseal from B&Q a while back and sure i paid about a tenner for a roll as below.