The MPAN 13 digit number is on the second line in the above image, with the 8 digit Meter number on the top.
To clarify, a Meter Point Administration Number also known as MPAN, Supply Number or S-Number is a 21 digit reference, used in Great Britain to uniquely identify electricity supply points. The gas equivalent is the Meter Point Reference Number (MPRN).
Although the name suggests that an MPAN refers to a particular meter, an MPAN can have several meters associated with it, or indeed none.
An MPAN is commonly separated into two sections, the core and the supplementary data. The core is the final 13 digits and is the unique identifier. The supplementary data gives information about the characteristics of the supply.
The full MPAN is required to be depicted on electricity bills (the boxes on the top and bottom line are generally unaligned).