This question has been touched on previously, but not exactly as I need advice.
I am deleting messages from my Sent Mail folder. Some of these are replies to messages that I have archived. If I delete the reply message, will the archived message be deleted? My impression from other responses is that it will and that to avoid this I have to select each reply and delete only that message - this seems to be a pretty time-consuming way to go about things!
The default is to delete the entire conversation. If you're hitting the big "Delete" button near the top of the page then that's what is happening.
However, it is possible to delete individual messages from a conversation. While viewing the conversation you need to open the menu for the individual message. (It's the down arrow next to the reply button.) From there you can delete an individual message from a conversation.
As you mention, that's going to be labor-intensive. You're probably better off turning off conversation view altogether if you're going to be doing this regularly (as mhoran suggests).
If you get a reply to your sent mail, and gmail recognizes it as being a part of the same conversation, then gmail will link the messages into a conversation.
To delete the sent message do one the following:
The big question is why? With 10+ GBytes of space and growing for your account, there is almost never a reason to delete messages/conversations to conserve disk space.
