What should I do if I re-read my last paragraph and realise I have moved 'off topic' from the original question?

This is a very common occurence, one which can be rectified relatively easily as long as you realise before the end of the paper. In such cases re-reading the paragraph you have just written can actually trigger your memory into understanding why you wrote what you did - it may be relevant but confused - in this case a clear, concise ending sentence to draw the material back to the question can be enough.

If you read back the paragraph however, and it doesn't make much sense to put it in the essay you can solve this in multiple ways; one such way would be to cross it out. Examiners do not want to waste time trying to understand why you wrote said paragraph, nor do they want to try to link it to the question. Another option could be to use connective words that give the illusion of a structured and coherent argument, such as "this therefore gives the..." or "thus" or "in light of the aforementioned point".

As a more pre-planned approach, doing lots of past papers and practising essay plans in 5 minutes and 10 minute time scales will really help to battle the "oh no, i didn't effectively plan and now i have gone off topic with only five minutes left" sounds!