AS Geoff described, following a recommendation from the Top Salaries Review Body in 1988, from 1991 all former Prime Ministers were entitled to a pension equal to half their ministerial salary, immediately on leaving office and however long they had served (worth £66,000 pa in 2009).
Since 1991 they have also received a special additional allowance- the 'Public Duties Cost Allowance' - to help fund an office, secretarial support and for work including answering letters and attending public events.
The maximum allowance for each former prime minister has steadily increased from £47,568 in 1997 to £100,205 in 2008, I couldn't find the information for the current rate but it would appear Thatcher is claiming the maximum available
A government car and driver for all ex-Prime Ministers was made available from 1975 so presumably id not included in these 'expenses'

Margaret Thatcher's ill health severely limits her engagements (in 2008 Carol Thatcher confirmed that her mother was suffering from dementia)
Her doctors advised that she should not make public speeches in the wake of some minor strokes six years ago Since then she has attended an address by the Pope in the UK and a party to mark former Defence Secretary Liam Fox's 50th birthday at his London apartment and claimed £535,000 of taxpayers' money from the public duties cost allowance available to ex-PMs.

That must be an awful lot of letters then!