Julian ~ Consider the free Weebly. Since it's browser-based, you'll be able to update your site from any modern Mac or PC. Weebly's net promoter score — an industry calculation of how likely users are to recommend a service to others — is an unusually high 80%. (Source)
You can get an idea of Weebly's capabilities by looking at their featured sites.
You can still re-design your site with iWeb. It will still work. It is an application on your Mac and if you have it you can still use it, but if you want an alternative, have a look at RapidWeaver, Sandvox, Freeway Pro/Exprress, Flux 4, Goldfish, PageSpinner, WordPress.org and .com, Drupal and Joomla.
All of the above are good alternatives and you can download free trials too. WordPress, Drupal and Joomla are all content management systems and you need a host/server where you need to install and run these from rather than being individual web design programmes.