Thanks for all the replies. It's been interesting seeing the various solutions that people have come up with.

As an update, I've now taken a left turn and migrated completely to Omnifocus on both the Mac and iPhone. I was running Windows on a Mac, which aren't the best bedfellows, so it made sense to use the Mac wherever possible.

It took a little while to get used to Omnifocus but now I find it does everything I need, both at the computer and on the move.