Hello I am a solicitor with over 15 years experience. I will try to help you with this.
This seems to be a very straight forward breach of contract in a business context. The verbal agreement and the letter sof promise almost certainly amount to a binding contract. How much do you think this man owes you?
We think that he owes us at least 20,000 It could be a lot more because this was a major work by a very famous artist. It can only be confirmed by seeing the bill of sale which he will certainly not show to us.
Ok you should instruct a solicitor in the orthodox way. This level of loss (£20,000 at the least and very likely a lot more) is outside the scope of the DIY small claims court. A miss-step could prove costly at this stage. The usual procedure would be to send a letter before action setting out your grievance and demanding that he tell you how much it was sold for and that he pay yuth emoney due. It would also contain a threat to issue proceedings if he did not comply with your demands. Although you could do this yourself it is much safer to use a solicitor. I would have thought you would be able to find a solicitor to do either a free or fixed costs initial meeting to check properly whether you have a claim or not. You should follow this link to the Law Society find a solicitor section http://solicitors.lawsociety.org.uk/
The tone of this man's replies and his obvious delaying tactics ring alarm bells in our minds.
All the more reason to get "lawyered up" then.
Before I finished.....Can I go straight to court without corresponding further with him?
No you should send him a letter before action to give him a chance to pay up before you start a court case but this will have more power if it is written by a solicitor.