The preseed url in the tftp file was incorect for me:
http://localhost:8443/unattended/provision

I found another ticket which said that it was worked out from the URL you visit foreman on when clicking "Build".

Ideally I guess this isn't right - I might be accessing foreman from a different network than the host being built.

In any case, visiting foreman as https://192.168.122.1 changed the URL to http://192.168.122.1/unattended/provision - still not https.

The attached patch fixes this - not sure it's the right way (I very rarely write ruby or rails!)

The preseed itself isn't working so I need to look at that, but at least this fixes one issue :-)