Dolph said:
Standard IANAL disclaimer.
In the UK, it would generally be inadmissable. Even the police have to jump through a load of legal hoops to get such evidence accepted.
As I understand it, it's a complex area. My first point would be that the rules that apply aren't entirely consistent across the UK. Some of the legislation is UK legislation, but Scotland has some variations.
The second point is that the whole area of phones taps is a minefield of contradictory and unclear law. There are rights
for the authorities to tap phones, provided they go through the necessary procedures, but there are certainly also cases where evidence has been produced in court despite procedural problems and that stance upheld by the Court of Appeal. So how does a Court weigh the authority's rights under RIP to intercept (with authorisation) against the individual's expectation to privacy under the ECHR? There's a conflict.
As for private individuals tapping someone else's phone, I would say it is categorically a breach of licence condition which, arguably, is a criminal offence (though arguably it isn't, but a civil matter), but I can't see how it possibly isn't a criminal offence under the Interception of Communications Act. That, basically, makes it a criminal offence to tap someones phone unless you fall into one of the specified exemptions and a private investigator wouldn't, unless he could somehow convince a court that he believed the person being tapped had consented to it. Unlikely, I'd have thought.
So, there are two questions :-
1) Would evidence obtained this way be admitted. Answer .... maybe. It's complex.
2) Would the investiagor be opening himself to criminal prosecution if he did this. Almost certainly, yes.