'Fraud' used to be dealt with under the Theft Act 1968. That provided the following defence:
"A person’s appropriation of property belonging to another is not to be regarded as dishonest—
(a)if he appropriates the property in the belief that he has in law the right to deprive the other of it, on behalf of himself or of a third person; or
(b)if he appropriates the property in the belief that he would have the other’s consent if the other knew of the appropriation and the circumstances of it"
Under that Act, simply pointing out that you were only getting the money which your ex had agreed you were entitled to would seem to have provided a sufficient defence.
However the Fraud Act 2006 (which, as far as fraud is concerned, replaced the relevant sections of the Theft Act 1968) provides no such statutory defence. However it does state that, for a successful prosecution, it must be shown that you 'intended to make a gain' (or intended to cause another person a loss). If you can show to a court that you had no such intention (since you were only reclaiming money which was rightfully yours anyway, rather than 'making a gain') you should not be convicted:
http://www.legislatio...35/crossheading/fraud
I think that it's definitely time to seek some professional legal advice!
Chris