Wouldn’t it be 20% of his 20% commission only? I think that's right, but correct me if I'm wrong. Whichever, the seller will lose 20% of the sale to commission - If the vat paid was 20% of the sold figure, the seller would be losing 40% after the commision, so that can't be right.
All the best,
Len
P.s - Makes me really think about parting with my copy though, as I have a Corporation Tax Bill I can't afford coming my way....