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You or your mum need to contact the Department for Work and Pensions, particularly the Pension Service/Pensions Direct, to find out what's going on. All you'll need is your mum's National Insurance number and they should be able to assist.
Phone number for Pensions Direct - 0845 301 301 1 (N.B. it's chargeable)
In the event that they refer back to NI you'll need to contact the Department for Social Development.
Hope that assists but please feel free to ask any follow-up questions that occur.