Use the Junctions and Services option to narrow down your search to a particular section of the motorway. M1 Hotels will then show you matching results that lie on the motorway between the locations you selected.
The first and last junctions of the motorway are selected initially, so you'll see all matching hotels by default. You can choose any pair of junctions/service areas. Where a service area is situated slightly off a junction, the junction is listed above it; otherwise they are ordered geographically.
Finally, you can of course choose a specific junction or service area for your search, by making the same selection in the two menus.