Upgrade fibre boost slow

Hi all

         Just upgrades to 76meg fibre boost £5 a month  , but only

          getting 44meg  had 37meg on fibre medium .So a few

           questions

          1/ Have I a problem with my line ?

          2/ Is 44meg the maximum speed my line will work at ? only 

              400 meters from the cab

           3/  If 44meg is the maximum speed how do I cancel this

                boost ? Says you can opt in or out a month at a time

                but can not see an uncheck button in my account/boosts

                      Thanks for the help in advance