Why not send the guy an invoice for £1000 assuming that he has decided to keep the machine. Send it by special delivery and make sure that you put clear terms on the invoice, (E.g. payment must be received within 14 days). The advantage of this is that, should he withhold payment again, you can now chase him through the small claims court which costs you nothing.
Once the small claims court finds in your favour, you can send the bailiffs round to enforce the order. He will have to pay them £1000 + their (usually exhorbatant) costs. Or else, they will seize goods to the 'auction value' of £1000 + their costs. Which will mean in reality the guy will probably lose £5000 worth of goods, or more! (This can be very satisfying to the innocent party
)
People usually pay when faced with the court order.
Hope you get it resolved!
PS If you have any emails, letters, postings etc Keep them safe. If you made the deal verbally, write down the precise date and time of the conversation and exactly what was said by both parties. Do this as soon as you can and keep it safe.