Statutory Maternity Pay claim

I have an employee with only one name (registered with the NI department with one name as her surname) who is eligible for the statutory maternity pay.
When we attempted to claim the advance for the SMP online in the usual way, we failed as the HMRC online helpdesk said that their system is only designed for people with both first name and surname and we should contact the HMRC employer helpline for further instructions.
We have been trying to reach them for a long time now, no answer.
Can anyone help please?

Hi tt210,
In this instance, you could put a full stop in the first name box. 
If however, you come across any errors in submitting this, please enter the surname in both the first name and surname boxes. 
Thank you