I've spoken to 123-reg. On their standard shared hosting plans which were purchased prior to 30/03/2015, the max_execution_time is set to 30seconds, and they refuse to change it on the basis it could cause other users on the same server to have problems. They have advised me that the new packages bought since this date have the max_execution_time value set to 120s.
I'm in the process of getting my hosting moved over to the newer platform and will report back what happens. Unfortunately it's not the 300 seconds as recommended by PrestaShop here.