You will need to hold a shareholders' meeting (called a 'general meeting' in the Companies Acts) if you wish to take any decisions which require the approval of the shareholders. This might include such things as removing a director, issuing new shares, or changing the company's articles of association.
In addition, shareholders with at least 5 per cent (or in some limited circumstances, 10 per cent) of the company's voting shares can require the board to call a general meeting for a specific purpose.
Shareholders can also pass decisions by 'written resolution in lieu of a meeting', provided they do so in accordance with new rules set out in the Companies Act 2006. These are more complex than the previous rules - take advice.