Ok Maggie, I've got a bit more info for you

When your Mary was working there, the hospital was run by Sisters of the Community of St John Baptist whose 'HQ' was at Clewer near Windsor. They have since relocated.
The work, and the name, carry on at the St Mary's Road site under 'new management'. St John's Home (a nursing home for the elderly) and other good things are now run by All Saints Sisters of the Poor - see their website
www.asspoxford.org for contact details. I suggest you ask them initially about the St John's records as this is outside the NHS, so not part of the Health Archive at the Oxfordshire Records Office.
Hope this helps,
Carol (great-niece of a Clewer Sister)