A word out of place might have earned him a nasty beating. But with a miniature camera in his clothing, one in his car and two wired-up colleagues in the crowd, all the angles would have been covered.
Donal produced gold dust - a graphic account of the way the drugs trade had suborned "security" at many night-clubs. The bouncers frisking clubbers for illegal drugs were swallowing amphetamine, cocaine and steroids delivered to their doors by courier. Almost a year after Leah Betts's death, the man who had hired Donal as a doorman sold him strong amphetamine with the words: "If anyone dies, don't mention my name."
Was this achievable in any other way? Open access was out of the question. Witnesses were too scared. And documentary evidence? You must be joking.
It is only recently that the technology became affordable, durable and reliable enough to mount an operation like this. It is a powerful new weapon for investigative journalism. The drawback is that this stuff is capable of ending privacy for ever.
With the technology comes subterfuge and further ethical dilemmas. Donal had to adopt another personality and identity for a whole year. Other World in Action journalists have posed as insurance clients, care-home staff and arms industry salesmen - all, we believe, to serious purpose and in the public interest.
It is not, of course, good taste alone which keeps World In Action, The Cook Report and Dispatches from filming, for example, "love romps". Independent TV journalists work to rules enforced by one statutory body, the ITC, and second-guessed by another, the Broadcasting Standards Commission.
The ITC Code is clear on secret recording. It may only happen when the story is of important public interest, and when it is essential for the authority and credibility of the journalism. Permission must be obtained from senior programme executives before filming, and again during editing. It is clear to me what this means - covert recording must be justifiable and it should be used as proof and not as mere illustration. I won't use it if we can prove our point without it.
The Broadcasting Code is unlike anything which print journalists are used to. It is legally enforceable. From its good intentions, a horde of Rottweiler lawyers and PR firms have fashioned a quasi-legal system in which TV investigations generate letters, legal threats and demands for prior disclosure. At best, this process keeps us on our toes, at worst it delivers the legal equivalent of road rage. Its great achievement, though, is in keeping us accountable and honest.
Into this secure environment, the Broadcasting Standards Commission has just thrown a large and pungent smoke grenade - a separate, much more heavy-handed code of conduct, produced at the behest of the last government. This is still in draft form, and it will not have the legal force of the ITC Code. It will, however, be the benchmark by which the BSC will conduct its hearings.
This draft code is seriously flawed. Sometimes it's just plain pedantic - like requiring every library shot to be labelled as such, without considering whether this always matters or whether the context might already make it clear. Its requirements on approaching interviewees would drive journalism further into a legalised paper-chase.
On secret filming, it is at its most restrictive. It would limit the use of secret recording to cases of "overwhelming" public interest. It defines these as crime and "significant" anti-social behaviour. The only exception would be some consumer testing. All those, of course, are valid. But they cannot be the only grounds for permitting secret filming.
Last February, an undercover World In Action reporter found herself in front of a principal fund-raiser for the Conservative Party. She was promised informal access to named Cabinet ministers for a minimum pounds 2,000 donation to election funds. This is not illegal, and I'm not convinced that it is "significantly anti-social" in the same way as drug-dealing or child abuse. But it told us a lot about the world of influence and money in the government party.
A year before, two covert World in Action journalists were in Hong Kong listening to someone describe the way she "knocks off" other people's designs and sells them on to Marks & Spencer. Can we equate this with drug-dealing or crime? But doesn't it tell us, legitimately, a lot about the world of fashion and its ethics? What, too, of the behaviour of auction houses captured by Dispatches or of Roger Cook's expose of "canned" animal hunting? In the public interest, certainly. But the overwhelming public interest?
In effect, we are now facing two separate codes, the legally enforceable ITC version, second-guessed by more restrictive BSC rules. This is double jeopardy and worse - for observance of the codes is no guarantee against the law of libel.
TV regulation of this kind was once necessary because of the scarcity of channels and the disproportionate influence of the broadcasters. In a multi-channel world, that is no longer the case. Now there is a serious imbalance in the opposite direction.
Dispatches, with an audience of two million at best, is overseen by two statutory bodiand a legally enforceable code. News of the World, readership 11 million, is not. Which is most likely to offend against taste, privacy and fairness? The writer is editor of `World In Action'.Reuse content