Thanks very much for your replies.
I'd seen the schedule for 5N 799 which showed flights for August, but yesterday I couldn't get the Nordavia system to find any flights on which I could book a ticket, whether for August or for next week. I had tried last month and had been able to book tickets for flights three months ahead, but no further - so I thought that there was some kind of weird three-month restriction on bookings (that's when I sent Nordavia an enquiry about booking ahead). However, yesterday, I couldn't get the system to find flights for any dates - as Marassa said, it probably was not working properly.
Interestingly, I tried again this morning on the Nordavia site and I could do a dummy booking for flights up to the end of May - but not for any time beyond that.
I tried the tutu.ru site but it only showed indirect flights via Moscow for our date - we're hoping to do the direct flight if we can so that we don't have to backtrack (and it does seem to exist!).
I suppose that I'll just have to keep trying the website at random occasions and hope that it works - or change our itinerary.
Thanks again for your help!