This ship belongs to a civilisation known as The System. They were humanoids who's population was linked in directly to a massive computer system - kinda like a cross between the Borg and the Bynars. The Pursuit Ship is a small assault ship they launched to recommandeer the Lberator back from Blake and his crew in the episode called 'Redemption" which opened the second season of Blake's 7. The System were destroyed, along with the Liberator's sister ship DSV2 , by Orac (a super-computer that Blake had snatched from the hands of Servalan and the Federation).
I've thrown them amongst the Roms, mostly because they can launch 'Plasma Bolts' and fire beams that look like Rom Disruptors.
MajorRacal