each button would link to a page passing a querystring variable:
<a href=""page.php?ContentType=1">Consumer</a>
<a href=""page.php?ContentType=2">Housing Associations</a>
<a href=""page.php?ContentType=3">Installers</a>
the on page.php use the set session variable server behavior to set the session using the ContentType querystring.
First go to the bindings panel and click the plus button, select URL Variable and name it ContentType.
Then on the server behavior panel, click the plus button and select eCart -> General -> Set Session Variable. For the trigger, click the lightning bolt, and select the ContentType URL Variable, do the same for the value, and set the name to ContentType