visitor visa for mother

Dear Forum members,
I plan to apply for my mother to visit UK for six months.
Her previous visa history:
She has travelled to UK 3 times in past (2008, 2009 and 2011). Each time she stayed approximately 3 months.
Forth time she and my dad (My dad first time) applied together. Both of their visas were refused stating that they intend to spend more than their monthly income on this visit which make their intentions doubtful to leave Uk at the end of their visa. They provided £40000 bank statement in joint account; they have objection for this much savings stating how come they have saved this much money. They both were Govt. employees and did provide payslips, NOC from their departments, Leave approval etc. but their applications were turned down.

Current Situation:
My mum is retired now and my dad has passed away. My mum is getting pension from her employer as well as from my dad employers. What documents she should provide to support her application?
Many thanks in advance
Meesha