My n'lock is very likely the oldest on the forum. It has the slight sideways play that everyone has; harmless, and you soon stop noticing it. It doesn't have the up and down motion you find, Richard.
My first thought was that some part of your headset has more than the essential play in it. The headset is normally set up by tightening it so you can just feel a slight resistance when you turn the handlebars and then backing off the nut a little. You can check whether the headset is torqued up properly by holding the front wheel firmly between your knees, pulling the brake hard on, pushing down hard on the handlebars and pulling forward (towards you if you have the wheel between your knees). If you feel a rocking motion or hear a knock, the headset needs tightening.
The other thing that might give a little vertical movement tangible through the handlebars is a loose front axle, held in tight enough for less than strenuous operation by the lawyer's lips, so you could check the axle nuts for the right torque. Doesn't seem all that likely to me, unless the front end took a knock somewhere, but it is possible.
If the unexplained motion you feel becomes worse, you need to stop riding the bike until you isolate and fix the cause.
Don't forget to report back when you've fixed the problem.
Good luck.
