COMPOSER_HOME environment variable error on running an update

Hi,

I'm still getting used to Statamic 3, sorry if I've missed this is the documentation.

From within the Statamic Control Panel on my live website. I tried to run the 3.0.3 update and ran into the following error…

In Factory.php line 648:

  The HOME or COMPOSER_HOME environment variable must be set for composer to   
  run correctly

I'm guessing this might be an environment varible is missing, but not sure what?

Thanks in advance
Grant