Welcome to LFS, by the way james. I noticed you've just joined up in the last few days
I've opted for saving ALL replays automatically. It's not a good idea if you don't have a pretty big hard drive, or if you don't add it to your list of "things I must do to my PC monthly". If you want to save your replays by default, go to Options > Game. The first two options are the ones to look at. Click to cycle through the 3 options available.
If you don't read your PC bedtime stories and otherwise pander to your PC's every whim and wish etc, then this might not be the best option. One thing to remember about manually saving replays is that, once you've saved a replay in a session, that's it... there's no more recording until either a new race starts, or you exit and rejoin the server. In other words don't save the replay until you're absolutely sure you've got as much as you'll want/need in it.