Exchange 2007 OWA access

Hi,

I have had the owa working for months on Exchange 2007. The problem I have is I want to swap ISP’s. When I run via my old ISP (BT) the owa works perfectly. When I switch over to my new ISP (NTL Cable), the Exchange server works in that it sends and recieves mail, but we can’t access the owa from external.
I have configured External DNS to the new IP address, but still nothing. I’ve checked the router and both routers have exactly the same ports forwarded.

Any ideas?