Thanks for all the comments - been really helpful.

Few thoughts on pre-GTD and where I am now:
- to answer one question, I hated the old to-do list because I didn't use it at all well - i.e. putting big projects on there, not next actions. That probably left me averse to relying on lists in future.
- and the other thing I hated was that it led to my time being double-counted: the to-do list would contain work I'm committed too, but at the same time my calendar would become completely full up with meetings and the like. 99% was an exaggeration, but it has often felt like I've had almost no time to actually work through a list of actions, hence why I wanted them on the calendar.
- so putting more tasks on my calendar was a noble effort, but misguided when the tasks weren't things that had to be done AT that time. Led to them being constantly moved, which made the calendar less sacrosanct.
- and to answer another point, my NA list has definitely been a mixture of Next Actions and Someday/Maybe items. I liked the fact, in a way, that I could review it regularly but not have to actually do any of the items straightaway. But that led to quite a few Somedays creeping in, which weakened the whole list.

Realising all that means I'll try making far more use of NAs in future, blocking out time for them, if only to stop that time being filled by meetings. The biggest problem was that I simply wasn't using that list properly at all. That alone accounts for 99% of the problems I've had with GTD.

Thanks again!