Andy, by law any VAT registered seller has to charge the going rate of VAT when selling any trade goods that are VAT rated, whether he has bought them from a private individual or a trade source.
IE he buys a bloggit from a man in the street for £10 sells it for £20+£3.50VAT he sends the IR £3.50.
Next he buys a bloggit from Withom for £10+£1.75VAT, sells it for £20+£3.50VAT and sends the IR the difference between the input and output tax of £1.75. If the new buyer is VAT registered and sells it on then VAT is chargeable again and so on until it ends up with a non VAT registered person.