Allows say I've got a domain http://www.foobar.com

underneath this document root are two sites: foo &lifier bar

I needed to produce a rewrite rule to ensure that anybody just likely to http://www.foobar.com is rerouted to http://www.foobar.com/foo

however it could leave anybody likely to other real sites alone.

so for instance:

http://www.foobar.com -> http://www.foobar.com/foo
http://www.foobar.com/bar -> http://www.foobar.com/bar

I'd think getting something similar to this:

RewriteEngine On
RewriteRule ^$  foo/ [L]

works however it does not

any help here?

Presuming it's inside your vHost config you would like:

RewriteEngine On
RewriteRule ^/$  /foo/ [L]