Bought a used car, do I need to pay previous owner for the road tax disc?

I've just bought a used car from a garage. I am due to pick it up at the weekend, but they have called me to say I need to pay the previous owner £77 for the remaining road tax left on the disc. Is this right? When I test drove the car it had the tax disc in and I assumed it would just come with it?
Update: There is 4 months left on the current disc
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