Help with understanding order of events

I have been helping my mother file her self-assessment after her tax adviser retired, I myself only have experience of PAYE, and I am a little confused about SE. I think I have two questions: 1. Is it better to wait until the tax bill (apparently in the form of two payments on account) comes in? I ask because she paid almost immediately on line, then it said she had overpaid (by a sum substantially more than the amount she had just paid) but also had a bill for two payments on account (so settling the amount for the first payment only involved a small amount which she has now done). 2.Can she ask HMRC not to bill her using payments on account (that is, just pay what is due once she has done the return)? It would be much easier for us to just file the return for the year just gone and pay it.

Hi,
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