Director general of the IIB Andrew Paddick has welcomed NIG managing director John Carrier's undertaking that the company will maintain its special relationship with brokers.
He said brokers would be pleased to see NIG continue to survive with a strong parent company.
"I am pleased with the statement that NIG will be retaining its autonomy," he said. "Brokers have been concerned at the possibility that NIG might disappear, so we should be glad that it will continue with a stronger parent company."
AiiB chairman Mike Slack was also satisfied with assurances from NIG's new owners over client confidentiality. He said: "Winterthur's philosophy is to develop in both direct and indirect markets. They have told me there will be no conflict of interest."
Mr Slack said direct writers were under increasing pressure to enter the broker channel because of the burgeoning cost of their massive advertising budgets.
But some brokers were more unsettled by the longer-term prospects of the takeover.
John Calvert of brokers Calvert and Co in Barrow-in-Furness, Cumbria, said: "There are fears in the back of brokers' minds that Churchill will make NIG's rates uncompetitive."
He accepted Churchill chairman Martin Long's assurances that the two insurers will remain separate and NIG will not give details of its clients to Churchill. But he stressed: "Churchill's chairman has personally guaranteed that brokers' client ownership will remain unaffected. I am sure what he says is right, but what happens after he steps down?"
However, BIBA chief executive Mike Williams described such concerns as "undue pessimism".
He said NIG's performance has been broker-friendly in the past and there was no evidence that its new parent company would change this.