hey Martyn,
as already mentioned in previous posts, it is a hardware problem you are experiencing. If you use an interface which plugs into the headset conector, it will lead to massive crosstalk.
Therefore you should use an interface which plugs into the 30- pin dock connector.
Check this Thread for further info.
http://www.shortscale.org/forum/viewtopic.php?t=44934
kind regards
Ludwig