Right well that gives us something to go on then.
I would suggest that you write a letter, setting out the full history. Ie what was agreed, the basis, the amount, the approximate dates. State that as it was agreed between you that in consideration of the payment of £20k the house would become legally and beneficially yours, he is now holding the legal title on your behalf as trustee. Unless he executes and returns the documents within 28 days you will be issuing proceedings against him which will include a claim for costs.