I have this now. (I omitted a negative sign)
If I rotate the dipole by θ from equilibrium then I've applied a torque:
τ = p x E, or
τ = 2qa.E.sin(θ)
The restoring torque due to the (uniform) electric field will be to reduce θ (and thus restore equilibrium)
τ = 2qa.E.sin(-θ) =...
My question is at the bottom of this post
PREAMBLE:
If a dipole is turned by an angle θ (in a uniform electric field) then the torque applied on the dipole by the electric field will be:
τ = 2.q.a.E.sin(-θ) = -2.q.a.E.sin(θ)
with the negative sign referring to it being a "restoring" torque...