I fear I may have dropped a big bollock and need some reassuring words from anyone involved in land / property. I purchased a piece of land with a few buildings on it last year and during the process agreed the land boundary on the registry plan was correct with my solicitor. Unfortunately, I never went into 1 of the buildings and didn’t notice the registry plan actually cut through the southern part of that building. The building doesn’t include any internal partitioning and the only way into it is from the part I definately ‘own’. I dont think there was anything ‘underhand’ going on by the vendor and it is just an honest mistake on all parts. So how stuffed am I ?

You should be ok. The LR can do rectification. It may need the vendor to swear a statutory declaration in support of your application. Speak to your conveyancing solicitor.