Robin,
attendance allowance (now known as Disabled Living Allowance) comes from the Dept of work and Pensions, nothing to do with Social Services. It's quite possible that Social Services don't know anything about your neighbour. If she has no-one to care for her, then you need to inform social services immediately, but be aware that they will probably put her into a home or hospital, at least temporarily.
The dept of work and pensions will send out forms for reassessment, but someone (you? or a relative/carer will need to fill them in).
Agree that contacting your neighbour's GP might be useful, or community nurse. Often these professionals aren't aware of a deterioration in thrir patients condition, just cos they don't visit often enough.
Does your neighbour have relatives. They need to be involved at this stage I think