Got the following from Silversea today:
"We have a charter scheduled with Enter Air, this is the same airline as last year. We don’t have the configuration, but the aircraft is expected with 180 passengers. A meal is provided with drinks but the exact menu is not available. We expect the plane to be full and middle seats will not be blocked. If needed, additional seats will be provided with SAS on a scheduled flight."
So here's what I found with some further online investigating...
Enter Air is a reputable charter airline based in Poland. According to their website, they operate 737 aircraft with 189 seats. When you google 737 with 189 seats - it's the same as TUI. Which means a 28 inch pitch with non-reclining seats.
I think it's going to be the longest 2:55 flight I've experienced in a long, long time! Luckily only in one direction and we have the Arctic waiting for us on arrival!
Looking forward to hearing what others have to say of their experience and I will be sure to report back on how ours went!