3 visas since summer 2016. A little more paperwork than ideal but smooth admin. Relax and go with it. Points:
A letter of invitation is important but easy. All hotels or tour operators will provide, often by return email. Get them to confirm which ref to use on the visa application. If you have not booked a hotel before applying for the visa a local company will send a generic invite to the city. Google will find one. A little leap of faith required. We did that for a white nights week in St Petersburg.
I used the Manchester office. In and out the building inside 15 minutes. Passport and visa by registered mail inside 7 days. London is busier but very well organised; have used them for my two work visas.
On arrival you need to register with the local immigration office. Sounds daunting but hotels, even the small lodging houses will do this for you. Check with them before you go.
One note of caution if you plan to tour independently ; you should reregister in each town.
I also only gave details of current plus last expired passport and only listed visits with passport stamps plus a couple of Euro holidays with approximate dates.
I declared full mil history including Kosovo. No problems or follow up questions.
They now also want more family details and info about social media accounts. That I refused to give with suitable selection of the 'correct' drop down option.
St Petersburg is a great city. We were there the week of the England vs Russia football but despite all the anti-Russian media frenzy the locals were without exception friendly and good humoured. Expensive by Russian standards but still very reasonable if you avoid the trendy clubs with black mercs parked nearby. Food and drink superb, everywhere immaculate and enough culture for an Activia addict. Have fun.