You can't do a SanCopy of the live LUN, it must be off a snap. If when the SAN snap executes the VM's have not all done their own snap you will only be crash consistent.Some people create scripts to initiate snaps on the VMs on the LUN before the SAN snap, others just live with the crash consistency.

Unfortunately the live data is on the CX300 and therefore no snaps available; I shall have to organise downtime to migrate it to it's new home on the CX500, this is not unexpected

What I mean by 'best solution' is the best I can reasonably achieve with this current set of hardware and tools, you have touched on what I was thinking with the use of scripts, snaps and san copy based on the CX500 to push to the CX300 at the remote site as the DR device. I highly suspect that once I have this working, and can point out it's short comings, I will be able to wrangle budget for something more fit-for-purpose like Vizioncores' offerings.