I've just finished the original campaign for the first time and have installed SOU, but are you supposed to just take the same character into SOU directly or start a new one. It seems odd that if I import my character I start with a level 15 sorceror, but if I start a new character I start at level 1. Does the game ramp up the difficulty if you start with a higher lever character or would I be starting at such a high level that I could do the first half of SOU blindfolded?

Start a new character - the OC and SOU aren't supposed to be linked by the same character. Alternately, you could always 'port a lower level character of your OC version in.

The game compensates for your starting level to some extent, but not as much as you'd like. A level 2, 3 or perhaps even 4 character might have an edge without the game losing its challenge entirely, but a level 15 character is sure to make short work of most of the SoU campaign.

Besides, from a purely story point of view, why would a lvl 15 char being going to adventurer school?

Good point.

So how come the final speech doesn't mention something like "Well done, thanks. Now off you go to the retirement home..."

Ah well back to school for me. Is it the same for HoU?

HotU was designed with the character you'd have by the end of SoU in mind. Coincidentally, your level 15 sorceror would fit the range perfectly as well.