rent arrears and tenant not for leaving

Rent Arrears

hi
i have a tenant who has occupied one of my houses for approx 12 months
she has very rarely paid her rent on time and usually let it go 2-3 months before bringing it up to date,her reason being that she is claiming housing benefit and is waiting for her claim to be sorted
she is now currently 6 months in arrears, i have issued section 21 which has now expired but when i say she has to leave she informs me she still has not had a decision from housing benefit
i have tried contacting housing benefit but they refuse to discuss any details with me about the case
please can anyone help as i am new to this and don't have a clue what to do next
thanks in advance