Um - semi-solved.
It came defaulted to categories disabled.
I selected Enabled, and saved.
Then I seleced Disabled, and saved.
My stuff reappeared.
I then re-enabled categories and tried to add a category.
Error - "There was a problem adding the category to the database."
I'm wondering if there needs to be another column in the database?
Unfortunately I don't know this stuff well enough to implement my guesses.
I'm going to copy out my todo data, disable the plugin, delete the database entries, and then re-enable the plugin - which I'm hoping will recreate the database stuff with all the correct settings and then re-enter my data.
My big concern was not losing the data, which I didn't. However, I would like to be able to take advantage of all the power in the new plugin - the category stuff etc seems very useful.