It didn't take her long to go behind his back after he'd dumped her ! The seed of dishonesty and deceit was already there,,as was the need to do damage,,and do damage she did.
Risking damage to herself as well.
Admitting the caravan break in. Without anyone even asking her about it.
Risking being called a lier & jealous former lover. Something people still accuse her of, even though Jeremy was convicted. She has had to leave the country.
Risking a perjury charge (if lying) & the courts bringing up the 1984 bank fraud.
Why ? She was not jilted. She could have played it dumb & said she knew nothing. But instead mentioned Matthew Mcdonald even though she would have no idea whether he had an alibi or not.