I have just checked the propertypriceregister website and copied their explanation as follows.
Prices Given Not Full Market Price
In a small number of transactions included in the Register the price shown does not represent the full market price of the property concerned for a variety of reasons. For example, the price declared may reflect the retention of an interest in the property by the previous owner, or the fact that a part or fraction only of the property is being purchased; alternatively, the property may have been purchased at a reduced price under the Affordable Homes Scheme. In addition, in a very small number of cases, properties may be declared as purchased in exchange for other property, stocks and shares, etc. All such properties are marked **.