git pull works remotely but not locally

Dear All

I have just set up my first git repository, and I'm trying things out before using it for real. To start with I'm using a centralised workflow with the main repos on a server, and workers checking work out and in. I'm using the smart http backend. I can contact the repos remotely via http or locally using the filesystem. However, git pull is not working in the local case. Please can someone offer help with (a) file permissions, and (b) difference between git pull, and git fetch; git merge (which works):

The basics seem to be working OK remotely: ie contacting the server over http:

         git clone http://username@12.34.56.78/git/projectname.git

What I mean is push and pull are working.

When working on the server itself I'm using

         git clone  /var/www/git/projectname.git

Most things seem to work the same, but git pull does not work:

         $ git pull
         fatal: cannot exec 'git-pull': Permission denied

fetch followed by merge seems to have the desired effect, e.g.:

    $ git fetch
    remote: Counting objects: 5, done.
    remote: Compressing objects: 100% (2/2), done.
    remote: Total 3 (delta 1), reused 0 (delta 0)
    Unpacking objects: 100% (3/3), done.
     From /var/www/git/projectname
        6b40c1f..4ffb389  master     -> origin/master

    $ git merge origin
    Updating 6b40c1f..4ffb389
    Fast-forward
      notes.txt |    1 +
      1 files changed, 1 insertions(+), 0 deletions(-)

So, my questions are:

(a) Why isn't git pull working in the second case? What permissions am I missing? (b) Does git fetch; git merge origin do the same thing as git pull?

With thanks and best wishes

Ivan