Ah was just reading up on that now.. although it didn't fix it.
My code is now:
void SendOKpage(EthernetClient &client)
{
client.print(F(
"HTTP/1.1 200 OK\r\n"
"Content-Type: text/html\r\n"
"\r\n"
"<HTML>"
"<HEAD>..."
The page still loads straight to a blank white page, no authentication prompt, no web page..