In which decade did Robert Peel become the first 'Conservative' Prime Minister of the UK?

Sir Robert Peel, 2nd Baronet, (5 February 1788 - 2 July 1850) was a British statesman of the Conservative Party who served twice as Prime Minister of the United Kingdom (1834 - 1835 and 1841 - 1846) and twice as Home Secretary (1822 - 1827 and 1828-1830). He is regarded as the father of modern British policing and as one of the founders of the modern Conservative Party.

The Tory Ministry was a minority government and depended on Whig goodwill for its continued existence. Parliament was dissolved in December 1834 and a general election called. Voting took place in January and February 1835 and Peel's supporters gained around 100 seats, but this was not enough to give them a majority.

As his statement of policy at the general election of January 1835, Peel issued the Tamworth Manifesto. This document was the basis on which the modern Conservative Party was founded. In it Peel pledged that the Conservatives would endorse modest reform.

The Whigs formed a compact with Daniel O'Connell's Irish Radical members to repeatedly defeat the government on various bills. Eventually after only about 100 days in government Peel's ministry resigned out of frustration and the Whigs under Lord Melbourne returned to power. The only real achievement of Peel's first administration was a commission to review the governance of the Church of England. This ecclesiastical commission was the forerunner of the Church Commissioners.