Nothing wrong when driving it, but I do notice that with the radio off I can hear a faint knocking noise when I wobble the steering wheel left to right about an inch or so each way (engine off)

Likely cause? The noise reminds me of a worn ARB bush perhaps. Any links to uprated bushes would be cool.

Most likely just the backlash in the steering rack, or if excessive worn track rod ends.

Worn TRE easy to diagnose, check wheels move when the steering wheel is rocked. There should be no play.

I get this too Jason, had it for the last 30,000 miles or so and it's never got any worse or noticable when driving so I've left it well alone

Oh I am sure the wheels do not move when I do this little wobble. The steering wheel is proper light and I use very little force to make it do what it does. Track rod ends are sounding like culprits although they're not that old to be honest.

You asked for a link to uprated ARB bushes..

As you wish Front ARB PolyBushes - 18mm Bar. Fabia/Polo/Ibiza