Thanks for the quick response Marcos.
Attached screenshots show the two firewalls currently in use -as you ca nsee neither would have ben blocking 53535 traffic (the specific rule on the Vigor was a later addition)
The reason for the frustration is that this is an intermittent issue. The customer gets a pop up and 20 mins later when we jump on their machine to investigate, the message has disappeared and ESET is happy.
The cycle then continues for various machines across all sites.
If it were a firewall issue, access would be blocked and it would NEVER work
But this seems like an issue with the LiveGrid servers themselves