As Deejay describes, St. Malvern's dance is a meduim-frequent rythmic wheel wobble that is transfered to the steering wheel, resembling what you sometimes can see at a wheel on a shopping cart. Most often in a straight line, around 80 km/h. A steering damper might help, but you are lucky if it totally eliminates it. BRBs will not affect this, but it will make the front end calmer and give a more stable feel, strongly recommended - easy and cheap. But it will not solve the problem.
Bump steer is when road wheel movement due to uneven road conditions is transfered to the steering wheel. This is pronounced on a Roadster S1, due to the tie rod geometry specific to that model. But it is dependant on the road, should not be present if the tarmac is slick and even, and it should not be a rythmic wobble. A steering damper is a good remedy for this, but again, lucky if it disappears.
As others have said, check the easy and obvious before you turn to more advanced surgery.
Are your wheels balanced? King pins OK? Wheel bearings worn or not? What about your damper blades? Road wheel alignment?
You might also check out the effect of fitting steering bearings,of which I have no personal experience.
