In a nutshell:

2 secure sites with their own SSL certificates.

I have moved from one host to another and now need to re-install these two certificates on a different server.

Is this possible or will they only work on the machine on which their original CSR was generated?

I'm trying to use the WHM to install them on a new IP and although it reports success I get 404s when I use https://...

Indie

You can move a cert from one server to another - as long as you have both the cert and the original server's private key. If you have to generate a new private key, your existing cert won't work and you'll have to get it reissued - or get a new one.

Thanks jimroe - much appreciated.