Job is via an agency who have been told not to worry about the salary, they will pay for the right person.  Even if this question was on an application form, I would still have the same question as I do now - what would I have put down?

They must have some sort of band in mind but they want you to answer first - if I knew their band, I probably wouldn't be asking this question as I would have a guide as to what they have in mind.

I would not be prepared to use my current salary (agency rate) as a starting base as it is much lower than if I was employed by the company I currently work for.

The PA role is working with the CEO. 

I was in this position a few years back, named a salary at the high end of my personal band and got it. However, it could have gone wrong - how can you tell? 

Comparing similar jobs in a similar location is no guide really as they vary so much.