Include report output in body of report when calling from reportExecutionService.Render

I'm using reportExecutionService.Render to create a report, and the rendering works fine.  I can't figure out how to send an email with the report in the body -- MHTML is the right format to use, I would think, but I can't see how to get the report output into the email body.  I tried setting the MessageSourceType in the SendMail task to a FileConnection, and pointing it at the MHTML output of the report, but that didn't work.  Is it possible to render the report into a variable and have the mail pick that up?

I'm looking for the same sort of result that you get with a subscription, but for various reasons I can't use a straightforward SSRS subscription here but have to build it on the fly.

I'm fairly new to the SSRS development environment so apologies if the answer is obvious.

Thanks

Mike