You've just answered your own question. What I read of the Statute of limitations, it isn't specific to a particular crime, but to the severity of it. EG: You knowingly claimed benefits you weren't entitled to, or you pulled off a Bernie Madoff level Pyramid scam.
If you wrong claimed a few hundred quid 20 years ago, I doubt the courts would bother. But if it were a Madoff level charge, going back 30 years, judges have the discretion to set aside the limitations Act.
Essentially, what City are accused of isn't even illegal in UK Law, only in the PL's rules which they state are governed by UK Law.
So by definition, if the criminal statute of limitations of 6 months was applied to our charge, we'd have no case to answer.
If the Civil statute of limitations was applied, the PL's got 6 years.
The only way any of extending either any further is if a court deemed the charges to be serious enough to warrant a hearing at the crown court, where there isn't a statute of limitations because of the deemed severity of the crime or civil case.
I read a few detailed pieces on the Government's website & various scholarly legal publications which I'll post later. I'm just getting sorted for the match, so hipefully this is OK for now.
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https://en.m.wikipedia.org/wiki/Statute_of_limitations#:~:text=Unlike%20many%20countries%2C%20the%20United,the%20Magistrates'%20Courts%20Act%201980.