Hired for one job but given another

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Just seeking some advice, my brother recently obtained his class 2 hgv license and registered with an agency, he was told as he had no experience they'd start him off with van driving work, and as promised got him a couple of multi drop shifts doing home deliveries for a well known over-priced diy chain.

But once he arrived at the depot the person he was told to report to told him you've no experience and you aren't getting it on our vans so you'll be drivers mate.

Can they legally do this? he went along with it as it was the same pay and he needs the money but it's not what the agency sent him for. The obvious thing to do would be speak with his agency but he's decided to just not say anything and hope the employer don't either so the agency consider him to have some experience and put him onto class 2 work which is what he signed up for.

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I suppose that is the best way to look at it, thank you all for taking the time to reply.