hey well i just called the number on the website (under 'contact')
basically the woman who told me to do that was reallyyyyyyy nice, so maybe if you guys call you might get someone horrible.
but anyway I got my mums boyfriend to fill out the form, and we wrote a letter that said how he doesnt contribute and has a life outside of our house that he uses his money for (obviously we said this in a polite way). I didn't get a call back or anything but then today I went on my online application and everything had been processed, and it has put me as eligibile for a few hundred pounds as a grant (which i definitely would not have got if my mum's boyfriend were included)!!
I don't know if this is important but I had a few things going in my favour: he and my mum aren't married, and he only moved in a year a go. maybe if you have a stepdad whoes been living there for years then you might have more of a problem?!
anyway hope that helps!