Which pi/s do I need to install the codecs on just the one doing the recording or both that and any other on the network that might be used for playback? Is it a playback issue that the video and sound is choppy or does it initially get recorded like that if the codes are not enabled?
Codecs are needed for devices that playback. Devices that record (eg with TVHeadend) would not need them as they generally just store the original stream to disc
Even if they are saving/reading from an external disk? I installed them on both pis anyway just in case.
If the device on which you are playing a recording is a Pi, you need the codecs installed on that Pi.
If you are saving a recording to a HDD, you don’t need the codecs until you come to play the recordings.