the torsion bar on the rear suspention is knocking when going over bumps . Is it a Mot fail ?

Torsion bar? Do you mean the radius arms or the shock absorber assembly?
Sure it's not the subframe knocking, this is a well known source of knocking?
Subframe knock is usually down to bushes failing, plenty about this on this forum.

Could it be the anti-roll bar?

No, as the anti roll bar does not have bushes, most likely the rear subframe bushes. The bar is held in place by 6 bolts and a compressed clamp.
See if you can jack it up and on both sides and see if theres any play in the subframe bushes. Try a pry bar between the body and subframe under the wheel arch. The bushes are available from a2z in Dewsbury. £90 front and £63 rears. Its a big diy job.

The roll bar at the front has rubber bushes...which are a common source of knocking and a cheap easy fix