I think it depends largely on your lifestyle. If you're stingent the whole year around, then £600/month should be enough, but you'll feel restricted with what you can do.
Remember that you'll also have to pay accommodation fees up front at the beginning of every term, so your mother will presumably set up a direct debit to her account to pay for it, and give you the remainder.
I'd recommend you just went for the loan really. The rate of interest on the maintainance loan is the rate of inflation. There is no real interest on the loan. Even if you don't want it, the loan in profitable: you can just take it all and put it in a high interest bank account for the duration of your studies: you'll make a profit on it!