i need to run an nec PBX WITH sip trunks on the same physical network as the hosted yealinks, can anyone see any potential problems here.the NEC sip trunks will need to be NATed and far as i recall the yealinks don't but feel there may be a conflict with port5060 or possible handshaking problems

I suggest a separate public IP and a router good enough to handle the routing properly or if they can change the ports they are using. Your hosted carrier would have the same issue if they attempted the same thing.

Your looking for trouble.
Get a 2nd circuit

many thanks, will push for duel WAN addresses.