Help needed - Mod Rewrite/redirect for subdomains

Hi,

I'm in need of some assistance (read: please bail me out 'cos I'm
really stuck!)

My webhosts (www.1and1.co.uk) allow subdomains to be created so that
for instance

domain.co.uk/directory1 can be accessed through
subdomain1.domain.co.uk
domain.co.uk/directory2 can be accessed through
subdomian2.domain.co.uk etc

This works fine _except_ the pages can still be accessed via
domain.co.uk/directory

I've done some very basic mod rewrite stuff to replace
http://domain.co.uk with http://www.domain.co.uk and *.shtml to *.php
and the like but after this I get stuck.

What I'd like to do is create a .htaccess file, presumably on a per
directory/subdomain level to rewrite (or redirect?) requests for
domain.co.uk/directory/page.php to subdomain.domain.co.uk/page.php

On the face of it this should be achievable, but to be honest its
beyond me!

Any help would be greatly appreciated.

Mik Foggin