When you need a personal reference (letter of recommendation) to apply for a job, or for a position on a Masters programme, the obvious first choice would be to ask your personal adviser. In addition, you could also ask your tutor or one of your lecturers. What's important is that the person knows you well enough to provide a reasonable personal reference.
If you're on a joint honours degree you should ask a personal reference from your personal adviser in each relevant department.
Nevertheless, it is important that you first seek consent from your PA, and that you send him/her as much information as possible about the post or degree programme you're applying for. This includes your CV, your personal statement, information about the post, admission requirements, etc. Better still is to arrange a brief meeting to discuss everything.
The person who will write your reference letter is required to adhere to the relevant College guidelines and should ensure the truth and accuracy of facts provided in the reference, and take all facts on your file into account, even if this means that the letter would have to be bland, or even negative. If this turns out to be the case, your PA will most likely inform you about this and request a meeting with you about your suitability for the post or programme.