How do I map users with a samba share?

I have a Synology Diskstation with a published windows share. On both the server and the client (Ubuntu 12.04) I have user accounts with matching username and passwords. (myth/myth)

The share is mounted with:

//192.168.88.1/plex  /mnt/plex  cifs  username=myth,password=xxxx  0  0

I can create, read and delete files on the share. I can't edit or overwrite them.

They get created as:

-rw-rw-r-- 1 1026 users  725178368 Mar 18 13:12

The server (Synology DiskStation, running DSM4.1, their own flavour of Linux) has a user called myth. The client (Ubuntu 12.04) has a user called myth. Both account have identical passwords.

So, it seems somehow I need to map uid 1026 to the user myth? How do I do that?

smb.conf from the DiskStation:

[global]
    printcap name=cups
    winbind enum groups=yes
    security=user
    local master=no
    realm=*
    passdb backend=smbpasswd
    printing=cups
    winbind enum users=yes
    load printers=yes
    workgroup=WORKGROUP
[plex]
    invalid users=nobody,nobody
    valid users=nobody,guest,admin,myth,nobody
    comment="Storage Location For PlexMediaServer"
    path=/volume1/plex
    guest ok=yes
    browseable=yes
    fileindex=yes
    mediaindex=no
    edit synoacl=no
    enable recycle bin=yes
    recycle bin admin only=no
    hide unreadable=no
    ftp disable list=no
    ftp disable modify=no
    ftp disable download=no
    read list=nobody,guest,nobody
    write list=nobody,admin,myth,nobody
    writeable=yes