The story is covered in The Daily Telegraph newspaper too, and as a quality newspaper, the facts as you've reported will likely be 100% correct.
Sadly, there is no automatic right of home or personal defence under UK law. You are allowed to use a REASONABLE or PROPORTIONATE amount of force in defence, but the key words are reasonable / proportionate and how they are defined. In practice, it's a bunch of men and women sitting in a nice safe office in 9-5 hours saying that how you acted after being woken at 2 am by somebody who has already broken into your property is 'reasonable' or not.
There have been several calls over the years for an 'automatic right of defence' and that intruders should automatically give up their rights to personal safety when they enter another's property, especially if it is occupied and it's during the night. These have been vigorously resisted by The Human Rights lobby, and by the UK legal and justice profession. In the past, wounded housebreakers have actually sued householders for loss of earnings, pain & suffering etc if the response was allegedly 'disproportionate'. In such cases, being invariably poor people with no income (on paper anyway) they even used taxpayer provided legal aid funds to hire lawyers and bring civil or criminal court cases. they've even done it from prison in a few cases after being jailed for the housebreaking!
In practice, attitudes have changed a lot here in recent years. After firing a shotgun or other firearm at housebreakers, the householder will invariably and automatically be arrested 'on suspicion of ..... assault with a weapon, attempted murder' or whatever. He or she will have a hard time in the police station being interrogated for hours and their actions and motives being questioned. In most cases, charges will not be brought however. It will come down to actual circumstances. Shooting (or stabbing) someone in the back who has been frightened off and is running away will almost certainly result in prosecution of the householder as the threat to his/her life had already been removed.
Having an intruder and especially four as in this case in front of you and apparently moving towards you threateningly especially after giving them warning, will almost certainly see charges not being placed. In practice though, the householder will have to convince the police that they were frightened out of their wits, felt really threatened and any action was 100% self-defence, not a response to anger or an attempt at reprisal.
Even if the police decide to recommend prosecution (and remember many senior police officers are anti-gun ownership on principle here), the CPS (Criminal Prosecution Service) has to decide whether to take the case to court or not and there are various criteria to be met, key ones being a reasonable chance of success (better than 50%) in front of a jury, and whether attempting a prosecution is 'in the public interest'. Attitudes amongst the UK public (who provide the juries) and press etc have changed out of all recognition in such cases, and it must be an open and shut case of gratuitous reprisals or over-the-top violence being used by the householder before juries will convict now. Being seen to apparently harass honest citizens in favour of serial burglars or low-lifes is increasingly seen as not 'being in the public interest'making the CPS reluctant to take such cases to court.
Use of a shotgun or firearm (compared to grabbing a household appliance such as a kitchen knife) automatically makes such cases potentially a more serious matter and works against the householder. Other issues arise. Was it legally owned? Our FAC and SGC conditions say weapons must be kept secure and hidden when not in use, so did the householder go to a gun safe and get the weapon out after being disturbed, or was it being incorrectly / illegally stored loaded under the bed or leaning in a bedroom corner? Even if no prosecution takes place the householder's FAC or SGC will almost certainly be automatically revoked, all weapons seized by the police and there may be considerable delay and difficulty involving the courts to get restitution at a later date.
If you Google 'Tony martin shooting' or look at the Wikipedia entry for this case,
http://en.wikipedia.org/wiki/Tony_Martin_(farmer)
you'll see a lot of the issues here so far as the UK is concerned. Mr Martin was an eccentric who lived alone in a remote farmhouse and had threatened to kill burglars of whom he was a serial victim. As a result his SGC was revoked, but he bought a pump-gun illegally and killed a teenage intruder with it. The illegal ownership and his past history weighed heavily against him, but the key evidence was forensic ballistics that suggested he had fired the weapon unreasonably. However, over half UK citizens polled on this case reckon he should never have been jailed. There have been many demands off and on since for a 'Tony Martin right of defence law'.