Thanks again Vance, the chapter on the King's Bench was most informative.
Below is an extract from the UK National Archives that seems to confirm that the "custody" of defendants in Kings Bench cases was largely a legal fiction.
"The court of King's Bench had jurisdiction in civil pleas in cases brought against persons already in its custody. In the fifteenth century, if not earlier, litigants discovered the benefits of using King's Bench procedures in cases which would otherwise have been brought in the Court of Common Pleas or elsewhere; in consequence, the notion of the 'custody' became elastic. In 1452 it was ruled that a defendant on bail was 'in the custody of the marshal of the King's Bench' for the purposes of being sued in King's Bench. Thenceforward there developed a standard procedure whereby a plaintiff brought a 'bill of Middlesex', alleging a trespass within the court's geographical jurisdiction, and then, when process on that suit brought the defendant theoretically within custody, the defendant was sued for some other, allegedly genuine, matter. Thus, from the late fifteenth century onwards until 1832 (when the Uniformity of Process Act abolished the stratagem), the vast majority of persons alleged in preliminary process to be 'in the custody of the marshal' were in fact not; only those subsequently committed by rule or order of the court were really in custody."
https://discovery.nationalarchives.gov.uk/details/r/C1216