If I turn the bars slightly I can hear something rubbing or grinding. It sounds like plastic rather than metal, but I cannot for the life of me find what's causing it.

I can hear it when it's full lock, or when it's a slight movement.

Anybody have any ideas?

 

Have you checked that all your cables and wiring is ziptied out of the way, usually that's a common thing to rub.
Have a friend turn the bars for you while you inspect the bike from the front

 

Thanks very much for the advice, I'll get some help and try and find what it is. Wires would make sense for the noise. I'll let you know. Thanks.

 

Does it do it with slight pressure on the break leaver. In other words, could it be the pads your hearing?

 

There are dust seals for your stem bearings. Unlike your wheel bearings, these covers are more of a cap. They become bent over time and can catch/rub on the frame. That's another thing you can check.

 

Thanks guys, I've found the offending wire! As it's a break line and so moves when I steer, I'm not sure how to stop it rubbing. I can't tie it out of place because it needs to move so do I need to put something around it to stop it becoming damaged?

 

It needs to move with the forks. You could tie wrap it the triple tree maybe.
Make sure you still have full rotation and no interference before riding.

 

My '89 has a guide that clips onto one of the fairing mounts. The brake line runs through the guide, allowing it motion but keeping it from falling down against the forks. Perhaps you had one that broke off?

Looks something like this (but not exactly as this is an internet picture):