It just might. My original portal is at around -1800, 260, and I emerged in the Nether at -204, 18. So for some reason this is quite a bit away from where you'd expect the portal to be, and when I go back it decides there isn't one close enough and makes a new one.
So I think I'll need to destroy the portal on the Nether side, make my way to around -225, 32, which should be the correct co-ordinates, make a portal there and hopefully that'll take me back to my portal.
Before I can try that out though, I have to destroy the portal that's above the sea, and work out how to get home from there.
Thanks for that insight.