In 2016/17 (as of 1 February 2017) the FCO spent £57,500 on tablet devices.
In 2015/16 it spent £3,610.
In 2014/15 it spent £146,125.
The FCO does not keep a central record of figures for tablet purchases before 2014.
The FCO employs over 14,000 people in a worldwide network of nearly 270 diplomatic offices. There may have been additional purchases of tablet computers out of devolved budgets. This information is not held centrally and to collate it would incur disproportionate cost.
Expenditure by all Central Government departments on office equipment, including paper, dating back to financial year 2011-12 is published on GOV.UK here: https://www.gov.uk/government/collections/greening-government-commitments#annual-reports.