I asked this with support and they pointed me here.
The script provided by HubSpot causes issues with Vue and breaks it, preventing it from running.
It does not occur in Internet Explorer 10 or Edge so we have temporarily prevented the HubSpot script from running in Internet Explorer 11 to keep the site working.
We have however left it running on one page to look at the error for yourselves: https://www.elvanto.com/hubspot-test. I've also attached the error that appears in Internet Explorer 11's dev tools.
Let me know if you need anymore information