As far as I can tell, the "official guidance" is still just the law as it stands. I managed to find that, it's section 30 of the Crime and Courts Bill. Actually hasn't come into law yet, no Royal Assent given, so already the DM has gone wrong.
Anyway, what it does is make small changes to law that already existed and was perfectly good. As best I can make out, the law says the following:
1) If you have legitimate reason to believe that your life is in danger, you can defend yourself by any means necessary (common law self-defence).
2) If there is no legitimate reason to believe your life is in danger, but your property is, then you can still defend your property, but "reasonable force" must still be used. Simply put, your property isn't worth his life, so you can try to incapacitate him if you want, but there's no grounds for lethal action.
3) The amendments don't restrict what you can do in your garden either, since the law was already in place -- and merely says that the house is to be treated in the same way as outside.
4) The law also allows a certain amount of leeway for people who "just panicked" and over-reacted, since "a person acting for a legitimate purpose may not be able to weigh to a nicety the exact measure of any necessary action" (section 76, subsection 7, Criminal Justice and Immigration Act 2008).
5) In the end you are allowed to defend yourself, but not to attack when there was no threat (direct, imminent or potential) to your life or property. So, while you can chase a man in the street after he's just mugged you or robbed your house, if you do catch him the time when you can claim that you were "defending yourself" has long since past. Can still perform a citizen's arrest if you are able, but no disproportionate violence.
The law was already perfectly good before Chris Grayling stood up last year and made this announcement. The changes ultimately made are tiny and haven't had any effect, so far as I can tell, on what we already had, which worked perfectly well.