Can I just confirm that you do speak Russian - otherwise it is nigh impossible to get around by yourselves and IMHO it is madness to attempt to do so. Taxis are not even allowed in the port so you have to start by walking over a mile to the port gates, then you have to barter (in Russian) with the waiting taxis to arrange where you want to go to and how much to pay.........May I suggest going with a registered tour guide like Red October, DenRus or Alla-tour - small groups and you can customise their tours as you wish, and most importantly no need for a visa!
All the visa info you need is at http://www.discoverthebaltic.com/spb.
To answer you question, this is the website. http://www.visitrussia.org.uk/. Be aware it costs £95 for a single entry and £105 for multiple entry. As others have said it is too much hastle and easier to book a ships tour or a land based tour, which included a group visa at no extra cost.