1. No, your solicitor is not right. Not only were the agents who sold the property to you liable, but so too is the owner who represented to you he was selling it to you. So too is solicitor for the other side who sold less that he allegedly was selling. There is also a potential liability for the solicitor who was working for you that he did not spot the different titles. So don't believe this solicitor when he tells you that it is your fault. A person or their agent (auctioneer/estate agent and solicitor) cannot sell less than their sales information represents. This is an actionable misrepresentation. So you have a cause of action against the original seller and his agents. You may also have a cause of action in negligence against your own solicitor. At this stage what you should do is quantify your loss by buying the un-included piece of land fromo the Crown and then suing all parties for the costs of buying this additional piece which was not included. Be aware that all the agents, such as an estate agent or auctioneer, or a solicitor, will carry professional indemnity insurance, so they will not be paying themselves. Their insurers will!
2. At this stage, I would advise you to see a new solicitor who is experienced in professional negligence cases. Get them to get the original file from the solicitor who worked for you. Then send the original sale documentation given by the seller and the solicitor's file, to a barrister to advise on the way to proceed. You will essentially be getting an opinion on fraud, misrepresenation and negligence. You will then have to issue legal proceedings to recover your losses.
This has been really helpful. We did in fact have to buy the land from the Crown, our losses are in excess of £13,500. Does is make a difference when we purchased? It was in 2006 and only came to light when we came to sell. We have now completed but not without huge delays, additional costs and stress!