Redirect Loops and .htaccess

I just moved from a CentOS dedi to an Ubuntu VPS. The site is custom coded PHP.

Frontend works fine (including rewrite rules). The admin backend I can't get rewrite rules to work...

First error:

H00124: Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.

Then after using debug level:

AH00122: redirected from r->uri = /admin/index.php

The relevant bits of my htaccess are:

# mod_rewrite set:

Options +Includes

RewriteEngine on

# Administration
RewriteCond %{REQUEST_URI} ^(/+)admin/(.*)$
RewriteRule (.*) %{DOCUMENT_ROOT}/admin/index.php [L,QSA]

# Rewrite orther
RewriteCond %{REQUEST_URI} !^(/+)index.php(.*)$
RewriteCond %{REQUEST_URI} !^(/+)syscmd.php$
RewriteRule ^(.*)$ %{DOCUMENT_ROOT}/index.php?page=$1 [L,QSA]

# If Rewriting Failure, Show error message (Internal backup)
RewriteCond %{REQUEST_URI} !^(/+)index.php$
RewriteCond %{REQUEST_URI} !^(/+)syscmd.php$
RewriteRule (.*) \1 [F]

This was working fine on CentOS too.

Any ideas? I already tried adding the following as the first condition:

RewriteCond %{REQUEST_URI} !/admin/ [NC]

That stopped it rewriting /admin completely.

Thanks