The Open University has released a full-text search engine for UK Repositories called CORE.
Here's an excerpt from the announcement:
Open access research is now more accessible as JISC has developed a new search engine to help academics, students and the general public navigate papers held in the UK's open access repositories.
JISC has funded the Open University's Knowledge Media Institute (KMi) to create an innovative new search facility which searches not just the abstract but the full text of the article.
When researchers use current systems like Google Scholar to search academic papers they can find themselves denied access to the full article, particularly when subscription fees are required. They also typically have to search across a number of open access repositories or use searches that harvest data from different sources.
But now, using the Connecting Repositories tool or CORE, people can search the full text of items held in all 142 approved Open Access repositories.
Once they've found what they're looking for, the CORE system stores these downloads, so that people can still get access to the papers they have found useful even if the original repository is offline. . . .
Search CORE with your research question.