I don't know what kind of backend they use, but for whatever reason, you have to use an additional HTTP parameter force=true when using the SPARQL endpoint from external services:
http://data.open.ac.uk/sparql?force=true > select distinct ?Concept where {[] a ?Concept} > LIMIT 100 -- Lorenz Bühmann AKSW group, University of Leipzig Group: http://aksw.org - semantic web research center
