I've proved out that the problem lies with the split routing, by issuing the following commands :-
route add -host 168.63.129.16 -ifp hn1 [msdefaultGW]
route add -host [externalfixedIP] -ifp hn1 [msdefaultGW]
This is locking down that any packets going out of the box towards the MS probe IP and a fixed IP on the internet are routed through the 2nd interface.
This all works fine, except that I can't work with individual static routes I need any traffic that comes in on the 2nd NIC to go back out the 2nd NIC.