Must admit have never heard of these. The big names are Alla, DenRus and Red October and Anastasia. I have booked with Alla for our trip. Have been most impressed so far by speed and quality of responses from them.
If you type petersburg into the keyword box within "find a question" at the top of the screen you will get previous answers to your question. Unfortunately the enquiry facility on this site has developed a glitch so you can only view the first page of answers even though it does say there are 3 but you will find some useful information in these.
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