Update! I was speaking to a marine engineer whilst buying some 2-stroke...and apparently the Garmin unit I have (451s) has an internal antenna and doesn't actually use an external one.
I can only assume the antenna and wiring is there from a previous model.
So there we go, going to try it out tomorrow and see if it works without the antenna plugged in at all.
Going to feel a bit daft for not RTFM considering I've had it for about 3 weeks now and been scratching my head the whole time!