It's 55.

I got 1220 Surveys of the Neath's Roof for getting to 55 & the second Sighting of the Castles.

1220 SotNR cashed in for 14,700 Service to London. So at 3 Far Afield per action + one to start, one to end, and one to cash in (22 actions) we're looking at 668 StL per Action for a recon mission if you can get to 55 Far Afield. Not sure how far you have to get to get a third sighting and what the checks would look like