Not cheap at £420, but the transformation was worth it to me, especially as I intend to keep my SV long enough to justify the cost. I was so impressed I saved up and got a Maxton rear shock too (even more expensive), but it was the fork upgrade that made the biggest difference by far. There are a few different options they can provide; have a look here
http://www.esling.btinternet.co.uk/maxton/. Richard Adams at maxton is very helpful with any questions and answered numerous emails.