R1150GS ADV 'Notchy' steering - Advice please

Hi All,
I have a '02 1150GS ADV with 20k showing which has a distinct notch or step, felt through the bars, when the bars are turned either way from centre line.

I realise that it will probably be a problem with either the top yoke bearing or the ball joint, but which one should I start with?

I have done a search but that seems to suggest that either is unlikely being as there is no lateral play in the handlebars (yoke bearing) an no obvious up/down play in the ball joint.

So which would be the sensible place to start? Or have I missed another possibility? Please don't suggest taking to a dealer as I've just bought the bike and i'm skint, so the repair ideally needs to be DIY.

All (relevent) advice greatfully received
Thanks

Steering head bearings. Time to change. Simple enough job.

Bought privately i guess,so no dealer warranty?.
Lower ball joints last a lot longer than 20K miles.
Anything like cables,pipes etc snagging when turning the bars?.
Underneath the plastic covers on the top yoke are 14mm (i think) nuts holding the stanchions up.If you remove these and allow the stanchions to drop down,you should be able to turn the top yoke independently and check for notchiness.

Good advice and yep, bought privately so no-one to blame but myself...
I have checked for anything 'snagging' and found nowt, so will drop the forks off the top yoke and check for the 'notch'
I hope it is the top bearing, it would seem much easier to replace than the ball joint (from what other GSer's are posting)
Cheers