Issue With SSRS Emailing Reports As PDFs
Another part of this puzzle was that the files that weren't able to send were created in a temporary location where they would then be deleted.

This issue is now completely resolved - in brief (after several false starts such as checking file/folder permissions and discovering reports would be sent via subscription as pdf if they contained a graph and not a matrix); we ended up using SMTP (set up on SQL Server) to send the email from the SQL Server to Exchange, which would then handle the rest of the journey.

Thank you all for your suggestions.