Hi, if i buy a laptop to run my cubase 8.5,i will intend to only use headphones.
Do i need an external audio interface?or can i just use the headphone socket on the laptop.
Probably but… Stenberg recommends an ASIO compatible audio card for best performance and low latency.
Regards. 
It’ll work with onboard soundcard using ASIO4all, but if you want low latency you’ll probably end up getting an audio interface.
I suggest you just try it and see if you need one.
thanks.
latency only comes into effect if im using a midi keyboard,is that right?
Yes, any MIDI interaction between Cubase and other hardware will suffer from latency.