Preferences/General tab. Remove check mark from the box "Select the Current/Previous Collection during import".
That looks to be correct! I'm heading there in about half an hour and will double check it definitely works with our weird set up (and on LR4 - I can confirm it's fine on 5) - thank you so much! You've saved a lot of hassle
Edit: Just went over to my brother and he's still on LR4 - no option there. But a great resource for anyone with LR5! I'll go my weird route of having to manually sync then maybe.
If you are on 4.2, try updating to 4.4.1 (last issue). It's a long shot, but I don't remember when the change was made.