Borger (and his co-author Richard Norton-Taylor) appear capable of careful, well-researched work. But reread this particular article. It is pure stenography ("Foreign Secretary William Hague said this, European diplomats said that, unnamed British officials added something else ... .") All reported uncritically, as if they didn't think the story was worth critical examination. Almost as if the authors didn't believe the claims either, but felt (lacking countervailing evidence) there was no point making an issue of them. I've edited quite a few newspaper articles in my life; trust me, even normally good reporters make that kind of judgment call.

That 2003 story is [rather was] an informative article, a kind we rarely saw in U.S. msm coverage. It reported cherry-picking of intelligence, mistaken analysis of intelligence, and fabrication of intelligence, all done with the purpose of advancing an agenda. It does not report anything that was new news at the time it was published. The current article which you linked to reports what is being said today by "western diplomats". It may be absolutely correct but I am not ready to give it much credence unless I hear more compelling evidence. The story may also be the result of purposeful spin.
All emphasis is added.

"British special forces in Afghanistan have intercepted an Iranian shipment of rockets to the Taliban that would have allowed them to double the range of their attacks, western diplomats have said."

 At this point he still noted that the information is based on what has been said by government sources. Are there still elements within our country's governments and that of others who have agendas which would be aided by demonizing Iran? I think there are. I do not think that because Borger wrote about lies by officials in the past that we should unquestionably accept as fact stories that he is now simply passing on from other officials.
 Today Borger seems to be accepting as fact and without question what he is told about this cache of weapons. Much like most reporters first did regarding the information in Borgers 2003 story. By two thirds of the way through the story it is reported as fact.

"It is the largest confirmed shipment of Iranian arms to the Taliban since 2007, when armour-piercing bombs were found in a vehicle in the western province of Farah.

The rockets appear to be a version of the Russian "Grad" which is an unguided weapon intended to be fired in very high numbers from dedicated launchers [which the Taliban almost certainly do not have and could not hide] at area targets. Fired individually they would be almost useless for an attack on a defended position. From twelve miles they could not be expected to hit within a hundred yards of any point they were aimed at even if they had the launchers and aiming equipment they were intended to use. They would, though, be affective as a terrorist weapon aimed at random into cities or villages, just like a highly accurate guided weapon is when fired at a target within a city or village.