Alternatively notices of breaches, enforcement action may be registered. I'd check the local search for these as well.
It might also be worth checking whether the property is in a conservation areas, listed etc... or have other planning contraints on it (Article 4 Direction etc...). If you see something and you don't know what it means jus ask on here.
There may also be other forms such as Additional Enquiries which may be in a form like the SPIF or raised in correspondence by your solicitor. They may hold additional information.
If you don't have copies, ask for copies.
This may help determine whether your solicitor missed something or whether there may have been some misrepresentation/mistake going on from the seller or their solicitor's side.
I would be interested to know, depending on when the annexe was built, how the council suddenly came to know about it especially considering the location. Have you any idea how they found out?
Could they have been tipped off by enquiries of the seller (or even your solicitor during the conveyancing).
I'd suggest writing in formally (keep a copy) asking for a full explaination. If no joy then put a formal complaint in eriting asking for a full explaination and they should instigate their formal complaints procedure and notify you of when they are to respond by.
If you still get no joy then I'd contact the Law Society for advice. Usually they will tell you to approach your solicitor first and give them time to respond so best do it this way round.