Roger, The way we have approached this in the past was to go with separate TLD's using Magento. The main thing we did was to focus on a complete content rewrite for all pages including product descriptions to remove any possibility of duplicate content issues holding back the new sites and then set about localising with directories, webmaster tools and some low level link building. This seemed to do the trick in getting our .co.uk and .ie domains to rank above the older .com original site.