Russia tourist visa denied & embassy refuses to give reason

Here's the timeline of events.

1. Applied for my Russia visa at VFS London on October 5th.
2. Went for interview at London Embassy on October 10th.
3. Was asked for reason of going, I said, travel and to see a friend.
4. Gentleman asked me, is your friend Russian citizen, I said yes.
5. He took my airline return ticket copy and hotel reservation copy.
6. On October 10, I got my passport back and a stamp (see below). No visa.
7. I enquired from Embassy but they said they have no comment on this.

Here's the backstory of the events.

• I was going to see my friend from Nov 2 to Nov 11.
• I live and work in London for 4 years and a UK resident of 8 years.
• I am a Pakistani citizen. Lawful, perfect record everywhere I've lived.
• I have had visas for USA, Switzerland, Srilanka, Saudi Arabia, UK.
• I have never been denied a visa, let alone being denied without reason.
• I got my hotel reservation and a tourist voucher from Hotels Pro.
• I applied myself and did not use a travel agency.
• My intended purpose of stay was tourism as stated in the documents.
• The documents I supplied were the following:
  • Application form
  • Passport
  • British Resident Permit photocopy
  • Hotel reservation
  • Airline return ticket
  • Tourist voucher from hotels-pro.ru
  • The tourist voucher had the Hotel name on it.
  • The application form had the info from tourist voucher on it.
  • No errors in application form or data of travel dates. All consistent.

My friend told me the stamp just means documents were accepted on 5th. They think this is where the visa was supposed to be stamped, because there was a yellow post-it note attached on that page.

1. When would be appropriate to retry and what should be done differently?
2. Would a travel agency make more sense like VisitRussia or Russian National Tourist Office?