Unless your family is somehow to blame (negligence, for example), doesn't it fall on the fiscus to make sure they don't just pay frauds? And if they made this mistake, couldn't they be held resposnible for this?
Just seems unlikely to me that the fiscus can just give money to the wrong person and get away with it. Even if said person committed fraud, it is still the government making the mistake. "Yeah, we're terribly sorry for not actually making sure that the person claiming your tax returns was who he said he was, but you can suck it and lose out on quite a bit of money due to a mistake we made." Not sure what it's like in the UK, but that wouldn't hold up over here unless your family was somehow at fault.