Only an advance non refundable fixed fee for a hearing doesn't exactly leave room for flexibility. Surely it just acts as a retainer for the barrister to be paid regardless of whether they are required to attend a hearing or not? I find the statement rather misleading.
The flexibility of a fixed fee is that you know how much you are going to pay for that hearing. It is like buying a concert ticket only to find out on the day of the concert that you cannot go and cannot obtain a refund. Disappointing but at least you pay no more money.
A retainer is usually between solicitor and client where the solicitor accepts instructions to carry out work and has to continue to do so as a solicitor cannot usually end that retainer - only the client can.
There are certain circumstances where the solicitor can end the retainer such as when a client fails to pay bills or provide reasonable monies on account of fees, when a client misleads the solicitor causing a breakdown in the solicitor/client relationship or if the client gives instructions that are improper e.g. to lie or prepare a 'false document'.