Further info for other users in the future.Suport have come back and the reason this isn't working is that the Zoom integration needs to be added for this to work. You can check if this is enable in the event /account from the event admin home left menu, bottom of the list 'Integrations'. If missing it needs to be requested as this then gets passed to the integrations team.
As a work around I will probably ad a video element to my first session and just let that run for the whole event. This would just be in Zoom regular meeting window though and not in the event website.
Ok, to clear this one up for others who may view at a later date. The issue turned out to be that creating the Zoom links within Cvent for the session works fine. The recording link was failing to show post Zoom meeting. After more testing, I found the issue, simply ensure that the Zoom recording is stopped before the meeting is ended otherwise the recording link won't be available. Makes sense now as if the meeting is endeed Zoom is unaware that the recording has ended so no link generated ;)
I will add a warning instruction for the host in big bold letters.