Thanks for the tip, I managed to find it as you said. It was one screen north of the starting point in the Marshes where those two live "plants" are (the ones that either make a bridge for you to walk on or smack you - sorry forgot its name). I didn't know you had to search for the portal by pressing the "X" button and found it was just about below the tree trunk with the first "plant". I used the Astral Amulet as it mentioned with the portal appearing and I was back in Sprigg's home. Used the Chrono Cross and my previous characters from my first playthrough returned!

I finished the game finally and will probably not play the game for quite a long while - have lots others to go through