These excerpts taken from the BP 6-K filed May 22, 2008.
Payment of calls
31 Each member shall (subject to receiving at least fourteen days notice specifying the time or times and place of payment) pay to the Company at the time or times and place so specified the amount called on his shares. The joint holders of a share shall be jointly and severally liable to pay all calls in respect thereof. A call may be revoked or postponed as the Directors may determine.