The problem Ardun is the general opaqueness of the market. Tinsa is probably the best of the indices out there but it is still, I believe, lagging the real market by 12-24 months.
The official figures are laughable. I recall a time a while back that they were still reporting price increases even though everybody knew at that stage that the market had been falling for a couple of years.
One of the main problems is the historical data. Up until a few years ago it was common practice for part of a property transaction to be done under the table to avoid tax. A 200k sale for example would be declared to the authorities as a 150k sale, the remaining 50k would be paid separately.
My understanding is that this is now mostly a thing of the past however it still greatly distorts current figures. For example, let us say the property that was sold for 200k a few years ago is now sold,and registered as so for 120k. The actual fall in value would be 40% (bought 200k, sold 120k) however the official figures would only show a 20% fall (bought 150k, sold 120k).
Nobody, whatever they claim, really knows where the market is here.