Hi Elizabeth,
If any of the photographs are stuck to the glass, it's best not to try to remove them yourself. A professional photograph conservator has the right tools and a controlled humidity setup to safely separate them. Since you're in the UK, I'd recommend checking the Find a Conservator page on the Institute of Conservation, as I'm not as familiar with private practices there.
In the meantime, here are some links that might be useful:
Practical Strategies for the Care of Film and Glass Negatives
Archived Webinar: Photograph Care
YouTube Recording
Best,
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Kelly Liu
Conservation Technician
Los Angeles County Department of Arts and Culture
Los Angeles
United States
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