Thank you, I think I'm starting to understand. It's definitely much cheaper than I can see elsewhere, so I'm quite happy for a slower option.
So I guess I can take any Thameslink from STP-PBO with £47.90 fare, not just the one shown in the itinerary. Or I could indeed start my journey at anywhere in Zone 1?
And with the £44.90 fare, I guess it has found a journey from Kings Cross, that allows travel by tube. So the relevant Advance is cheaper because it doesn’t need to include a Zone 1 ticket? If I buy this ticket, I do have to start it at KGX, and travel somehow to Peterborough. But I could presumably travel by tube somehow from King’s Cross to St Pancras? I guess I can't just start at STP.
I also see a “London Underground and DLR Zone 2” to NCL option (same 1846 departure as above) for £48.30. Does this include my tube (but not Thameslink!) from Kentish Town/ Archway? There is also “London Underground and DLR Zones 1-2” at the same price.
Indeed “London Underground and DLR Zone 2” to NCL finds me a split on the direct 1930 train. Something which doesn’t appear if I search from KGX.
Sorry for all the questions! It's just a bit mind boggling, and they all look similar but are quite different.