The guard explained that the prisoner had been arrested under orders from the previous King, Henry II. But Henry died before it could be revealed what charges were being brought against the prisoner, so they continued to hold him in a judicial limbo.
Henry II died on July 6th, 1189. So that means this episode of 'Robin Hood' took place in the Spring of 1198. The dating for previous episodes could be determined from there, as well as the episodes that would follow.....