The code that I had used as my basis had worked fine with a clean install of WP2.8 but didn’t like widgets that had been created under WP2.7.

An hour later of googling & trawling through the source code and I was none the wiser. Then I noticed that WP_Widget automatically adds on ‘widget_’ as a prefix to the id_base property. I had used a different naming convention when I had built custom widgets (e.g. starting them with the project or client name, such as ‘redefine_’…).

If I set the option_name property so that it was just the id_base then all of my widgets successfully reappeared in the upgraded site – but not the default WP widgets. So I was halfway there.

Finally I stumbled across a reference to  an alt_option_name property for WP_Widget – that was the solution to my problem. I could set that to the id_base of each my custom widgets & leave the new WP_Widget class alone.

Hopefully this will be of use for other people who have also used a different naming convention for widgets on previous versions of WordPress.