Yes, afaia most estate agents are capable of putting a value on a garden, when the house was originally bought, that is later sold once they know the total value of the property. Again afaia revenue accept this figure.
When the remainder of the property is sold you can only use the remainder of the value as the cost, you can't get it revalued, revenue will pick up on this or at least they should. In other words both purchase values (the investment property and the new property now being sold) should add up to the actual purchase price of the original property.
Hope this makes sense.