CCI Debt collectors wanting more money!

dellaclearing

Hey everyone, sorry if I'm posting this in the wrong place. I've recently sent off an incoming / outgoing form to CCI debt collection agency for my unpaid final uni year's tuition fee. After going through the form I had £65 leftover monthly, so I offered to pay them £20 per month. I've just received a phone call from them asking why I'm only offering £20 instead of £65, I told her that I wanted to have some kind of a life as well (a whole £40 worth!) and that if I gave every spare penny to them I'd be sitting indoors until the debt was paid off. She left it there but said she'd need to speak to me again. Can they legally force me to part with everything I have left over? Any help or advice appreciated, thanks!