Ann Penston, a friend of Gloria Foster who died after her care agency was closed down, says she wants to know how the tragedy happened.
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The weapon was found by a member of the public in bushes in Greenford.
Production on the sixth film in the franchise, M:I 6: Mission Impossible, will be put on hold while he recovers.
The horse suffered a broken leg when it was hit by a HGV lorry and had to later be put down at the roadside.