Thinking a little about the planned shape of the series, I thought I'd share this.
Young Gwernin trilogy (published):
1. Storyteller - location: Wales; date: 550-551 CE; publication: 2007.
2. Flight of the Hawk - location: Wales, Northern England, Scotland; date: 551 CE; publication: 2007.
3. The Ash Spear - location: Wales, Northern England; date: 552 CE ; publication: 2009.
Gwernin's Quests trilogy (writing in progress):
4. The Druid's Son - location: Wales and Ireland; date: 559 CE ; publication: 2011.
5. The Well of Wisdom* - location: Wales and western England; date: c. 562 CE; publication: ?
6. The Green North - location: Wales, Northern England, Scotland; date: c. 567 CE; publication: ?
The Fall of the North trilogy (planning stage only):
7. The Last True King - location: Northern England and lowland Scotland; date: c. 573 CE; publication: ?
8. Three Hundred Spears* - location: Northern England and lowland Scotland; date: c. 577 CE; publication: ?
9. The Fall of the North - Northern England and lowland Scotland; date: c. 580 CE; publication: ?
*Titles on these two keep changing. The rest have been fixed for a while, but might change before publication.
I'm also planning on a prequel, but that's a story for another day.
-GRG