and eextract from this website is below and confirms that the grant must provide a minimum of 2.5hrs. You cannot be charged extra just because the grant does not cover what the nursery would normally charge for 2.5hrs.
It als echos what BB says, if the nursery fees are less than the grant they keep any excess.
The NEF is a fixed amount paid for every session a setting provides. For this amount the setting must provide a free 2½ hour session. The setting must provide the sessions free and must not charge and then reimburse the fund. If the cost of a session is less than the NEF amount paid, the setting may keep the extra money and use it to improve the provision for 3 and 4 year olds. If the cost of the session is more than the NEF amount paid, the NEF session must still be free and the remaining charges adjusted accordingly.