Overview
When the Bank introduced Money Market Reform in May 2006, the Bank ceased publication of M0 and instead began publishing series for Reserve Balances at the Bank of England to accompany Notes and Coin in circulation. For more information see the Notes and Coin and Reserves Balances.
In layman's terms, narrow money is approximately equal to cash in circulation. It is a measure of money which excludes several less liquid forms of asset (such as deposits at commercial banks), which do nonetheless contribute to broader money aggregates such as M4.