Complex sentence: The scheme which is designed as an open resource and is completely free to join has since developed into a national network of Heroes Welcome Towns and Cities where a special warm welcome is offered to members of the Armed Forces.
Split into simpler sentences: The scheme which is designed to encourage open support for members of the Armed Forces is an open resource and completely free to join. Since inception it has developed into a national network of Heroes Welcome Towns and Cities where a special warm welcome is offered to members of the Armed Forces.