The knob from my Lyriks is sitting somewhere round Ae. Can I buy any RS rebound knob to replace it?

Just use a 2.5mm allen key.

I could yeah but I'd rather have the adjuster knob.

I've done the same thing with a Reba.

It bothered me for a week or two, but now I'd rather have the £16.00 (yes, that's how much the crab-walking, toad-licking highwaymen at SRAM charge for a new one) in my pocket.

It's a bad design anyway – buy another and it'll only fall off again.
Think of it as a weight saving measure.

I used to take mine off to stop me faffing with the rebound whilst out riding…i like to tinker.

Not all interchangeable, some have shorter keys than others. If yours was a short key, check your LBS as they may have kept back some old ones if they've replaced damping cartridges under warranty on Dart/Toras.