The linked server has been set up to use a generic login rather than the login used by sql server. However, when I tried to run the sp as another user, (including myself!) I get the following error: (so not sure why that is happening - if I run it without use of the 'EXECUTE AS user =' line, it runs fine)

Msg 15274, Level 16, State 1, Line 2

Access to the remote server is denied because the current security context is not trusted.