It's funny, I guess MrBump was making the same assumption as me - "I've probably got a better chance of a straight line with sanding, even if takes two weeks of doing it every night after work..."
But what you're saying Wez is "probably not" to that assumption then? And in fact a "slowly and carefully with the sandpaper" is more likely to cause a bog up? Certainly fits with my experience now I think about it :lol: Previously, I'd just assumed that something unspecified had "gone wrong" and put up with it...
Unless you're sanding with a block - which will be tricky in a neck pocket - it's very difficult to sand evenly. If the paper just has your fingertips behind it, they're an extremely long way from being evenly shaped!
Sanding in a small, shallow space like a neck pocket, there's a natural tendency to sand more near the top edge and hardly at all at the bottom of the pocket - it's almost impossible
not to do that. So you end up with a neck that correctly fits the bottom of the pocket but has a gap all round at the top!
A router's the best bet - if you can get one and you're comfortable using it. Of course it's an extremely fine cut so
don't try to do it freehand, use a template! :P
(
Edit: Wez replied before me, so this a bit of a redundant comment... )