During an inquest into the death earlier this year the Oxleas NHS Foundation Trust, which oversees Green Parks House, said Mrs Mager's death "could not have been predicted or prevented".
But after court proceedings were served, the Trust admitted liability, saying they should have prevented her death.
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The 47-year-old man died shortly after 3pm on Tuesday.
Specialist explosive detection dogs searched through the town examining letter boxes and phone booths for signs of potential devices.
Temperatures reached 29.1C in St James's Park, central London.