[jaymajik]

THIS IS NOT LEGAL ADVICE SO DON'T RELY ON IT BUT SEEK YOUR OWN LEGAL ADVICE

but generally under English law:
- a company can reclaim money paid by mistake by suing the recipient for unjust enrichment.
- the recipient would likely have a defence to the company's claim if he had spent it while honestly believing it was his to keep and had changed his spending habits in reliance on having the extra money.
- if the recipient knew all along the money was paid by mistake, such a defence would not be available (as the recipient would not have "changed his position" in good faith). So if the recipient spent the money knowing it was paid by mistake, he would likely remain liable to a claim from the company.