I signed up at the outset of the business for Corporation Tax? and PAYE...
Then, when I tried to sign up for VAT recently, it wasn't having it. They have a specialist team answering the telephone purely dedicated to advising about the web-interface. This proved invaluable and they got me signed up with another account and I was able to file my VAT return the same day.
However... I now have two big Government Gateway ID's, which is a bit of a hassle. So I either work out how to merge the two, or seriously improve my organisation skills!
Online VAT is great, and the form calculates the sums for you. You also get 7 days extra to file your VAT return if you do it online I hear