You cannot get both the postgraduate loan and the social work bursary, so you're right there.
For the social work bursary, for post grads the basic rate grant is up to £3,362.50 per year if you are studying outside of London. In London, it increases, but only slightly to £3,762.50. The maintenance grant is up to £2,721 per year, £4,201 if you are studying in London. The Bursary also only pays £4,052 towards tuition fees, so if the course is more than that amount you will have to make up the outstanding cost yourself. If you drop out the course, you will be expected to pay back the amount as the bursary will not cover this for you. Usually you will be eligible for it in year two if you were for year one, but this may not always be the case dependant on the university and the number of applicants.