Russian Visas:
Russian Visa Online, Tourist, Business, Multiple-Entry, Travel Russia Visa Support Invitation Services.
Ask for the manager, Ekaterina. She is knowledgeable about crew visas for Russia.
Invitation letter done in 15 minutes, speaking about an EU (but non-UK) passport.
All other visas:
www.crewsolution.com. Ask for Michael. Expensive, but priceless if it is the only solution that can get you that freelance gig.
For Visas to North Korea, PM me!