Hilary, sorry for the delay getting back to you. We did, as predicted, run into trouble using qRules in the browser and had to revert to using a code solution based on this article: http://www.bizsupportonline.net/infopath2007/copy-rows-from-sharepoint-list-to-main-data-source.htm
I'm replying here in case it might prove useful to anybody else who has the same problem.
Thanks again for your help.
Nick.