The problem was with the ics Design file. I had the line at the top:
<MySource_PRINT id_name="__global__" var="content_attachment" extension="ics" />
which can be useful when viewing from a browser (as described here [Expert] Creating Individual Calendar Event iCal files – In 3 Easy Steps … ), but isn’t needed when attaching to a form email as you specify the file name and extension.
It seems that the ics extension was getting added to the Thank you page.
Thanks to Squiz Support for solving this for me