The Court of Appeal (Criminal Division) has given judgment in R v BA  EWCA Crim 1529 where the court considered a ruling by a trial judge (Mr Recorder Marrin QC) that a wife was not to be compelled to testify against her husband at his trial for an offence under section 2(a) of the Criminal Damage Act 1971 - "A person who without lawful excuse makes to another a threat, intending that that other would fear it would be carried out, - (a) to destroy or damage any property belonging to that other or a third person; or ...."
Section 80 of the Police and Criminal Evidence Act 1984 (PACE) was considered. However, the court declined an invitation to construe section 80 so as to remove anomalies and bring it more into line with modern thinking. That was a matter for Parliament.
A witness is said to be COMPETENT if he or she may be lawfully called to give evidence. A witness is COMPELLABLE if, being competent, he or she may be lawfully compelled by a court to give evidence.
As a starting point: (a) all witnesses (whatever their age) are competent to give evidence - Youth Justice and Criminal Evidence Act 1999 s.53(1) and (b) all competent witnesses are compellable.
Section 53 maintained the position that a person charged in criminal proceedings is not competent to give evidence in the proceedings for the prosecution. However, a "person charged in criminal proceedings" does not include a person who is not, or is no longer, liable to be convicted of any offence in the proceedings (whether as a result of pleading guilty or for any other reason).
Competence of Spouses (and civil partners):
Before the Police and Criminal Evidence Act 1984, the spouse of an accused was (with limited exceptions) incompetent as a witness for the prosecution. The Criminal Law Revision Committee's 11th Report (1972 Cmnd 4991) recommended changes which were eventually enacted in PACE section 80. Subsequently, PACE section 80 was amended by the Youth Justice and Criminal Evidence Act 1999 - (Schedule 4).
What follows is a basic summary of the modern position.
Competence - In any proceedings the wife or husband of the accused is competent to give evidence (a) for the prosecution; and (b) on behalf of the accused or any person jointly charged with the accused
Compellability for the defence -
In any proceedings the spouse or civil partner of a person charged in the proceedings is compellable to give evidence on behalf of that person - PACE 1984 s.80(2)
In any proceedings the spouse or civil partner of a person charged in the proceedings is compellable to give evidence on behalf of any other person charged in the proceedings but only in respect of any "specified offence" with which that other person is charged - PACE 1984 s80(2A)(a)
Specified offences - see sections 80(3) and 80(7) for definition of this term.
1] W is married to H. H is on trial for criminal damage (contrary to section 1 of the Criminal Damage Act 1971) and is jointly charged with N. If W so chose, she may give evidence on behalf of either or both the accused. She is competent to do so. W may not be compelled to testify on behalf of the prosecution since criminal damage is not a specified offence. However, W could be compelled to testify for the defence on behalf of H but, since criminal damage is not a specified offence, she may not be compelled to testify for the defence on behalf of N.
2] W is married to H. H is charged with an assault on C (aged 6) who is the child of H and W. W is competent to testify either for the prosecution or the defence. Here, W may be compelled to testify for the prosecution since the offence is a specified offence - see section 80(3)(a). W could be compelled to testify for the defence on behalf of H.
3] W is married to H. H is charged (jointly with D) with taking indecent photographs of a child - Protection of Children Act 1978 s.1. Again, W is competent to testify either for the prosecution or the defence. W may be compelled to testify for the prosecution since the offence is a specified offence - see sections 80(3)(b) and 80(7). W could also be compelled to testify for the defence on behalf of either H or D.
A number of difficulties continue to exist with PACE section 80. For one thing, the restriction on the compellability of witnesses only applies to spouses and civil partners. The rule will apply as long as the individuals are married or in civil partnership and this is so even if they are separated and going through divorce or dissolution proceedings. On the other hand, those who merely cohabit will not benefit from the rules no matter how close and permanent their relationship. It may be that, at some future date, this will be challenged under the Human Rights Act 1998.
There may also be an anomaly in cases where one spouse is charged with a series of offences against another spouse as the spouse is only compellable in relation to "specified offences." This may be a problem where the offences charged are part of the same course of conduct but only some of the offences are "specified."
Yet another issue is that a trial judge has a discretion under Criminal Justice Act 2003 s114(2) to admit a statement even where the witness is not compellable. On this, see R v L  EWCA Crim 973 at paras. 34-36.
The actual decision in R v BA
The meaning of the phrase - in PACE 1984 s80(3) - 'involves an assault on, or injury or a a threat of injury' - has been unclear. If (say) the offence charged is assault occasioning actual bodily harm then the offence would be a "specified offence." However, suppose the offence charged is a threat to damage property. Making such a threat may amount to an offence under section 2(a) of the Criminal Damage Act 1971 but is it a "specified offence" so that a spouse becomes compellable? In R v BA, the Court of Appeal held that the learned Recorder was correct in deciding that the offence was not a "specified offence."
The court stated (at para 20) - "Looking therefore at the policy, language and practical considerations in a broad and purposive way, we consider that Parliament decided to strike the balance by reference to the nature of the offence and not by reference to the factual circumstances surrounding the offence. The division in s.2 makes clear that the offence under s.2(a) is an offence directed at property where it cannot be said in such cases that the offence gives rise to the real possibility of an assault or injury or threat of injury. The Recorder was therefore right to rule that the evidence of the wife was not compellable."
It was for Parliament to consider the various anomalies thrown up by section 80 so as to "bring it more in line with modern thinking." "It is for Parliament to consider whether the present restrictions contained in the 1984 Act are apposite to contemporary society."