Hello Nicola, I'm Keith and happy to help you with your question. This is a real Dad's Army 'Don't Panic' situation.
If she invoices you for her casual services of course you can pay her on a self employed basis. It is the normal way self employed professionals operate one to another.
Her invoices will form vouchers for an expense within your account.
I do hope I have set your mind at rest on this tiny problem.
I have been reading on different forums etc about what other people have experienced with the tax office and the reason I am a bit unsure is that some have said that as I will potentially be paying someone to help me with a specific job then they should be employed and not self employed. If I can pay her as self employed then I do not need to let tax office know as she will sort out her own NI and tax etc??