A lot of today's blog and CMS systems are entirely database driven, and when you make an update to a page it is quite simple to simultaneously update your feed. It should be quite easy to update your feed when you update the data driven parts of your site.
The static HTML pages will be a bit more difficult. One solution for automatic updates would be to run a content scraper on those pages and generate the feed updates that way.
Just a supplementary question, I wondered if there was a program, or script available which would note that a static page had changed and generate a message that either the page itself had been amended, or in some way which part of the page had changed (news, new products etc.)
I have found plenty of info on converting a RSS feed to a static HTML page but not vice versa.
There are plenty of services that will monitor pages to tell you if there's been a change. Some of them will even give you that information in an RSS feed. You can search for "RSS feeds to sites without feeds", or terms like that to find several.
The problem is whether those services will make the feed for you. That's hard to say depending on your feed. You'd still have to take that information and add it to your feed. Like the scraper idea I mentioned earlier you're going to have to do some programming to get it to output what you want.