I often come across this as a solicitor. Usually I have to write a letter explaining the situation, then saying my client won't pay more than 75%. The problem for them is that if the LA tries to take her to court for the missing 25% the court won't be as fussy in wanting to get evidence of where he's moved to.
Of course it helps that my clients are seeking and paying for legal advice. That is usually given as the reason why the LA accepts the letter from me as proof.