The Law seems pretty reasonable to me, in terms of what constitutes an acceptable response, level of violence employed.
http://www.cps.gov.uk/publications/prosecution/householders.html
From the press report, it seems that the injuries were inflicted whilst Mr. Woodhouse was chasing the intruders.
From the CPS guidelines,
"What if I chase them as they run off?
This situation is different as you are no longer acting in self-defence and so the same degree of force may not be reasonable. However, you are still allowed to use reasonable force to recover your property and make a citizen's arrest. You should consider your own safety and, for example, whether the police have been called. A rugby tackle or a single blow would probably be reasonable. Acting out of malice and revenge with the intent of inflicting punishment through injury or death would not."
You could rightly question whether 2 broken legs and a broken arm constitutes reasonable force when chasing intruders, rather than defending oneself from attack.
For those of you who are questioning whether this householder has been treated fairly, what level of injury inflicted upon the intruders running away would you consider unacceptable, if any?