If you remember back to the Aer Lingus ticket fiasco from a few months ago, the only reason Aer Lingus caved in other than PR, was because a contract existed - they had taken payment.
A contract doesn't exist here. This is the same as you picking up an item in shop that has an incorrect price, you present it at the counter the cashier tells you the correct price. The shop is not obliged to accept the incorrect price, the insurer is not obliged to stand over a rough estimate provided by the agent.
As for threatening Joe Duffy, does anyone ever give in to this threat?