This has happened to me twice in the last 18 months. DSP paid up eventually.
Your agreement is with the tenant, not with the DSP. Your tenant still has an obligation to pay the rent. You should notify your tenant that they are in arrears and this is a breach of their obligations, that you are entitled to terminate the tenancy if the rent is not paid and give them a reasonable amount of time to pay the arrears. The initial notice that the tenant is in arrears does not need to be in writing but it might be advisable to do so. I consider 10 working days to be reasonable.