An easy way to split up one blog into many?

I’m redesigning my blog over at keithneilson.co.uk in order to separate the various things I write about into their own blogs.

I have chosen tho do this via the network feature and have a testing environment ready to go with blogs set up on my local machine. My issue is the following:

I cannot use the export/import option because the export function in WordPress is useless as it is missing posts and does not maintain the category hierarchy. Leaving it to export all posts exports everything up to January (last month) but nothing beyond that. Selecting a specific category to export is even worse as it seemingly loses posts at random.

Is there an easy way to export posts belonging to a specific category (and its children) that is more reliable and bug free than using the dashboard?

I’m willing to get my hands dirty in phpmyadmin if necessary but will need step by step instructions for both export and import.

I don’t want to simply use a straight DB backup as I’m not sure if it would work.