You can also load an Ordnance Survey map using Ordnance Survey's OpenSpace, currently based on OpenLayers v2.7, see the Ordnance Survey Open Space Version for a working example.
IE8 & OL - Due to a conflict, pages such as this which are standards compliant but contain an OL map, which should run in Standards View, actually don't - vectors (markers) are only displayed properly in Compatibility View. Here this is set by a <meta> tag.