The amount of funds required from members will vary, depending on the perceived level of risk in the business which they underwrite. This is known as "enhanced capital requirement" or ECR.
During each year the aggregate value of Funds at Lloyd's follow a reasonably predictable cycle, as a result of the practical process of adjusting individual members' funds to the changing shape of their underwriting business.
Source for all figures: Lloyd's Market Services, January 2009.