Ok thanks for the info. An alternative is to create a Google Form and use Google Apps Script (which is an extension of Javascript), which has a GMail object that allows you to do this. Then embed the Google Form in a Raw HTML includelet in e-voice. This also allows us to generate the email body content in a format different to that generated by e-voice Forms. I'll try it and let you know.
I tried the Google Forms approach. I replaced the previous e-voice Form with a link to a Google Form (opens in a new window). The Google Form has a trigger for OnFormSubmit that fires an Apps script handler that parses the form responses and generates an email to the site managers of just those sites selected in the checkbox. Its a bit tricky to set up but it does work ok. If anyone else needs functionality like this I can give details of how to do it. The form is on our "Apply for a plot" page. Many thanks.