For the purposes of HEFCE policy compliance the article must be deposited in Hydra, even if another repository is also used as well. This is to enable HEFCE to harvest relevant information from the repository as required.
Is this a local requirement? Reading the HEFCE policy I do not see a requirement to deposit in an institutional repository specifically (http://goo.gl/vteWGH):
Deposit requirements
17. The output must have been deposited in an institutional repository, a repository service shared between multiple institutions, or a subject repository such as arXiv .
18. The output must have been deposited as soon after the point of acceptance as possible, and no later than three months after this date (as given in the acceptance letter or e-mail from the publication to the author).
19. The output must have been deposited as the author’s accepted and final peer-reviewed text (which may otherwise be known as the ‘accepted author manuscript’ or ‘final author version’ or ‘post-print’), though this may be replaced or augmented with an updated peer-reviewed manuscript or the final published version of record at a later date
by Richard Poynder on Apr 28, 2015
We recognise that the HEFCE policy does not explicitly state that a local repository must be used. HEFCE has also stated it wishes to collect date on compliance. Hence, to ensure we can fully comply we need to hold a copy of that data. As such, our local open access policy requires that research outputs have to be deposited in Hydra, even if another repository is also used. We will aim to facilitate deposit in other repositories through Hydra or through other interoperability options over time to avoid any duplication of effort.
Deposit requirements
17. The output must have been deposited in an institutional repository, a repository service shared between multiple institutions, or a subject repository such as arXiv .
18. The output must have been deposited as soon after the point of acceptance as possible, and no later than three months after this date (as given in the acceptance letter or e-mail from the publication to the author).
19. The output must have been deposited as the author’s accepted and final peer-reviewed text (which may otherwise be known as the ‘accepted author manuscript’ or ‘final author version’ or ‘post-print’), though this may be replaced or augmented with an updated peer-reviewed manuscript or the final published version of record at a later date