Does anyone have any general advice on exam technique?

Number 1: Listen to instructions
Number 2: Read the instructions on the paper
Number 3: Read the question before you answer it.

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Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

Perfect Toadmum.
We had a huge sign saying “RTQ” Read The Question! in the classroom.

It is the main reason for wrong answers

Number 4: Answer it in the right place on the answer sheet.

Don’t spend too long on one question - if you can't do it, move on to the next one otherwise you may run out of time.

If all the questions are weighted equally then don't spend too long on one question when you could do 3 others in the same time.

Similarly, if one question awards 4 marks and requires a more detailed explanation, don't spend too long on short answers that award 1 mark.

Find out the format, how many questions there are in each section and how many minutes you get. Then child knows approx how much time to spend per question. Practice it.
During exam, make a note of start and finish time - my child said they were told how many minutes they had but no finish time up on a board.
If running out of time, go to the last few and answer the same for every question, eg put D for them all, to cut down on thinking time. This is especially true in NVR where it's almost impossible to answer the questions in the 30 seconds (10 questions in 5 minutes).