It is certainly in contravention of current Building Regulations, but BR are not retrospective.
Therefore it MAY be possible that it was not unlawful when it was done.
Either way its not a good idea and you may wish to factor this into your offer for the property - on the grounds you don't think it is safe.
Get a price for the work and then use it as leverage to get the cost of the house reduced. In the current climate you should be sucessful. I don't suppose the vendors have buyers queueing for the property.