Is the guarantor still liable?

Guarantor

The tenant has been in the property for about two years [originally 6 month RLA Ast form used]

9/3/18 the guarantor sent me a letter stating due to reitrement he no longer wished to be guarantor. I returned a letter saying I had recieved his letter and referred to point 6 Page 2 of AST. Also advised the guarantor due to the tenant changing to Universal credit there was rent arrears. Although I hoped this would get sorted out.

I issued a section 21 on 15/3/18 to the tenant to expire 20/5/18. Mainly to focus them into sorting things out. They have stayed in the house and I have not taken further action to enforce section 21.

The tenant has been paying the rent each month since the universal credit kicked in but over £800 is still owed. The tenant has offered to pay an extra £50 a month to clear the outstanding rent.

I am getting different advice or possibly mis-understanding it. Apparently the guaranor is still liable for arrears whilst she lives there on the existing ongoing tenancy? [unless I have agreed to him ending being guarantor]

So is the guarantor still liable to cover the unpaid rent?