The Sport England report referenced above (dated March 2008) can be found here. The extra water space figure used in the above diagram comes from the table in section 3.29 on page 45 of this report:

[From table] Additional swimming pool provision (4 lane x 25m and above) needed by 2016 based on population increase [alone] [...] St Albans: 1.9 [4 lane by 25m pool area equivalent] [...] Additional swimming pool provision (4 lane x 25m and above) needed by 2016 based on population increase and participation increase (5%) [...] St Albans: 2.3 [4 lane by 25m pool area equivalent] [...]