What your solicitor should have done was arrange a Transfer of Equity, that is the transfer of your share of the house to your boyfriend, and for the house to be remortgaged in your boyfriend's name only. However, the TofE can only take place if your boyfriend was willing and Nationwide agreed.(posted 8226 days ago)If the Nationwide had agreed and the Transfer took place, then you have no liability.
If the Nationwide didn't agree to the Transfer and you had walked away, even though you told them about your situation, it was still a joint mortgage and you were still liable regardless of whether or not the payments were up to date when you left.
As the house was only repossessed 3 years ago, its possible that the Nationwide might try and pursue you for the debt, even if you haven't been on the scene for over 12 years. Nationwide have probably been trying to trace and pursue your ex-boyfriend for the shortfall and haven't been successful, so they're trying you instead - that happens a lot when couples have split up.
If you think there might have been poor advice given by your solicitor then I would suggest that you ask for a copy of your file. Its possible that the firm still might have it. Depending on the exact written advice given, then you may have a claim against the solicitor. But thats something which can wait for now.
If you do get a letter from Nationwide or their solicitors demanding payment, then come back to this website and read up on what to do next, or post another message to this board.