20.4—(1) A defendant may make a counterclaim against a claimant by filing particulars of the counterclaim.
(2) A defendant may make a counterclaim against a claimant—
(a)without the court’s permission if he files it with his defence; or
(b)at any other time with the court’s permission.
(Part 15 makes provision for a defence to a claim and applies to a defence to a counterclaim by virtue of rule 20.3)
(3) Part 10 (acknowledgment of service) does not apply to a claimant who wishes to defend a counterclaim.