The Parliament Act of 1911 is often seen as the first historical landmark in the attempt to reform the House of Lords, but the 20 years from 1640 to 1660 probably saw the most radical changes ever in the membership, duties and nature of the upper house.
First, the 26 bishops were excluded from sitting in the House in 1642, followed by the most drastic "reform" ever - the abolition of the House of Lords, as legislated by the Commons in the Rump, from 1649 to 1660.
The Other House - life appointees
Perhaps more relevant to the more recent proposals for a reformed House of Lords than outright abolition is the project in the Humble Petition and Advice of 1657 for the Other House. This upper chamber was to consist of between 40 and 70 life appointees, nominated by the Lord Protector and to be approved by the Commons, who were to exercise the same legislative and judicial powers as the abolished House of Lords. Cromwell even styled his nominees, most of them commoners, "my Lords".
The Other House itself never came into any practical existence, as in January 1658 its establishment was blocked by those Members of the Commons who were to approve its membership and in Richard Cromwell's Parliament in early 1659 it was accepted by a slim majority of the Commons just weeks before the Parliament itself was dissolved.
The Other House only got as far as a "reform" of the House of Lords on paper, but its proposal suggests that there was a recognition of a need for a second chamber as a check on the power of the single lower chamber.
Since the demise of the Other House in 1659 there have been a host of other proposals to reform the House of Lords to maintain its function as a counterbalance to the Commons while removing some of the difficulties associated with its unelected and (until recently) hereditary nature.