The £5,000 you sold the van for will count as a disposal so this will be offset against the new van.
Claim £16,250 AIA for new van minus the £5,000 disposal so £11,250, you do not have to claim the whole amount if this is not beneficial and would mean losing some of your personal allowance.
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Doug
These are only my opinions of how I see things and therefore should not be taken as advice