Apologies for the delay in replying, I had some PTO the last few days. I had raised an SR with support around the same time I posted last week and have gotten feedback yesterday to say that what we have currently setup will not work as BGP on the ESGs evaluates the ASN also for ECMP so if different ASNs are used only the paths with the lowest ASN will be used. Resolution is to use the same ASN , KB article here outlines this:

VMware Knowledge Base

Bit of a surprising one, need to see if I can get this configuration on the physical side to confirm this works for us.

Cheers

DB