Just been through another game in which the Ally Elrond came up, and I got to wondering about his ability. The card text says;

Quote

Response: After Elrond enters play, choose one:

• heal all damage on a hero
• discard a Condition attachment
• each player draws 1 card.

Am I right in thinking that it would only apply to a Hero controlled by the same player as Elrond? I'm guessing that if it could be used to help any player in the game, it would specify 'Choose a Player'?

When a player card refers to just "a hero" (or "an ally", "a character", etc), it is not restricted to your heroes. The phrase "a hero you control" is used to signal such a restriction. Generally, the game is generous about letting you help out the rest of your fellowship, in keeping with the setting's theme.