This might help: https://www.plumbase.co.uk/potterton-velaire-vitesse-10000865-3560258/explodedviews
If you know the exact Model number if your boiler you could do a search using your favourite search engine for something like "Myson Model oil pump", replacing 'Model' with your own data, and see what comes up.
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Myson velaire oil pump?
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