I am persuaded by sojourner's answer that "further arrest" has a specific legal meaning under the British Police and Criminal Act (PACE) of 1984. I am unaware of any such meaning of "further arrest" under U.S. statutory law—certainly the term was not used in the context of criminal law in the courses on criminal procedure and evidence that I took in law school back in the late 1970s.
I am not, however, persuaded that "further arrested," under PACE, has the meaning that sojourner attributes to it:
further arrested is a legal term which means the extension of arrest time of a person already in police custody (first arrested) on the grounds of new evidence or new/ additional charge/charges.
My impression after looking at various instances of the phrase "further arrest" in PACE is that the term refers not to an extension of existing detention of a person already in custody but rather to a subsequent taking into custody of a person previously arrested and then released.
The two excerpts from PACE that sojourner's answer quotes (subsection 2.6 under "Elements of Arrest under section 24 PACE" and subsection 3.3 under "Information to be given on Arrest") appear to accommodate either interpretation of the phrase—as does the glancing mention in subsection 3.4:
3.4 A person who is is arrested, or further arrested, must be cautioned unless: [two specified exceptions omitted].
Also ambiguous are these subsections under section 10 ("Cautions") of Code C of PACE:
(a) When a caution must be given
10.3 A person who is arrested, or further arrested, must be informed at the time, if practicable, or if not, as soon as it becomes practicable thereafter, that they are under arrest and of the grounds and reasons for their arrest, see [cross-citation omitted].
10.4 As required by Code G, section 3, a person who is arrested, or further arrested, must be cautioned unless: (a) it is impracticable to do so by reason of their condition or behaviour at the time; (b) they have already been cautioned immediately prior to arrest as in paragraph 10.1.
(b) Terms of the cautions
10.9 When, despite being cautioned, a person fails to co-operate or to answer particular questions which may affect their immediate treatment, the person should be informed of any relevant consequences and that those consequences are not affected by the caution. Exampls are when a person's refusal to provide:
their names and addresses when charged may make them liable to detention;
particulars and information in accordance with a statutory requirement, e.g. under the Road Traffic Act 1988, may amount to an offence or make the person liable to a further arrest.
But two subsequent mentions of "further arrest" seem to me not to support the "already in police custody" interpretation. First, from section 42 ("Authorization of continued detention") of PACE:
(10) Where an officer has authorised the keeping of a person who has not been charged in detention under subsection (1) or (2) above, he [the arrested person] shall be released from detention, either on bail or without bail, not later than 36 hours after the relevant time, unless—(a) he has been charged with an offence; or (b) his continued detention is authorised or otherwise permitted in accordance with section 43 below.
(11) A person released under subsection (10) above shall not be re-arrested without a warrant for the offence for which he was previously arrested unless new evidence justifying a further arrest has comes to light since his release; but this subsection does not prevent an arrest under section 46A below.
Here, it can hardly be argued that the person "further arrested" after having been released under subsection 42(10) is "already in police custody"; it seems unmistakable that the "further arrest" refers to the person's being "re-arrested" on the basis of new evidence.
Second, and to very similar effect are provisions 45(18) and 45(19) under section 45 of PACE ("Warrants of further detention"):
(18) Where a warrant of further detention is issued, the person to whom it relates shall be released from police detention, either on bail or without bail, upon or before the expiry of the warrant unless he is charged.
(19) A person released under subsection (18) above shall not be re-arrested without a warrant for the offence for which he was previously arrested unless new evidence justifying a further arrest has comes to light since his release; but this subsection does not prevent an arrest under section 46A below.
It's unfortunate that PACE doesn't provide a specific definition of the term "further arrest," considering how often the words come up in the statute. But in view of the fact that sections 42(11) and 45(19) address specific situations where a person has been arrested and then released, it is impossible to interpret the words "further arrested" in those provisions as meaning "subjected to an extension of one's already existing detention." So either "further arrested" means something very similar to "re-arrested" throughout PACE or it has different meanings in different provisions of the statute—a most unfortunate attribute for a term with substantive legal meaning to have.
As I noted earlier, the term "further arrested" is not used in U.S. statutory law, as far as I know. Thanks to sojourner, we have ample evidence that it is used in UK law, though it appears not to be explicitly defined in PACE and consequently is subject to variable interpretation or misinterpretation by outsiders like sojourner and me.
Applying my interpretation of "further arrested" under PACE to the situation that WS2's question cites, I would say that "further arrested" would be appropriate language to use only if the man and woman "further arrested" on suspicion of murder had previously been arrested on suspicion of kidnapping but then released from custody prior to the "further arrest."