The form itself had imported but the VBA element had not.
Not having seen this error before I hit up Bing and soon found a blog post from Dex Master David Musgrave where he discusses this exact issue on GP9 (I was loading a customisation from GP9 to GP 2010 to upgrade).
The answer was not quite what I wanted. I was hoping for something nice and simple, but instead I needed to export all the customisations, delete the forms.dic, reports.dic and dynamics.vba files and then reimport all of the customisations.
After I did this the import worked fine.