I think parts of the volume decline can be explained by the following:
* the disappearance of buyers that don't intent to live in a property (buy-to-let, flipping).
* restrictions in the mortgage market means that upgrading to a higher value property requires more equity, therefore people need to stay in a house longer.
* due to uncertainty in the economy, buyers and sellers are very risk averse and reluctant to increase their monthly outgoings. They prefer to stay put.
Comparing sales volumes to the high of the property boom is not that useful, it won't reach similar levels until the next bubble. I'd think it is better to look at the long term trend over the last few decades.
My personal opinion is that property values will stay flat for the next decade or so. Inflation will take care of a decline in real terms.