You may ask, why do I think the "they" referred to are the security services? Is there any evidence that there is a single source, as opposed to a loosely based "whispering campaign" amongst many people? Even if there is a single source, is there any evidence that "they" are professional "buggers" as opposed to amateurs, or perhaps people working for a privately funded organization?
a) As to the question of a single source versus something more fragmented; it is quite obvious that there is a single source from the way the campaign has been carried out. Since things have been repeated verbatim which were said in my home, there must be one group which does the watching and listening. Since on several occasions (mainly during travel) people have been planted in close proximity and rehearsed in what they were to say, it follows that someone must have done the planning for that, and again a single source is indicated.
b) So why couldn't it be amateurs? Why couldn't it be a private organisation, for example a private detective agency paid to manage the campaign and undertake the technical aspects? Some detective agencies are unscrupulous as has been proved on the occasions in the past when they've been exposed or caught; they too can have access to the bugging technology deployed; and there are reported cases of MI5 paying private eyes to do their dirty work (against peace campaigners and similar enemies of the state) on the understanding that if they were caught then they could deny all knowledge. Why couldn't that be the case?
The main factor pointing to direct security service involvement (as opposed to amateurs or MI5 proxies) is the breadth of their access to the media in particular, and the fact that the television companies are so involved in the campaign. The BBC would not directly invade someone's home themselves, since it would not be within their remit to allocate personnel or financial resources to do so. An organisation of their stature would not take part in a campaign set up by private sources. The only people they would take material from would be the security services, presumably on the assumption that if the cat ever flew out of the bag yowling it would be MI5 who would take the consequences.
State sponsorship for these acts of psychological terrorism is also indicated by duration; support for over six years for a team of three or four people would be beyond the means and will of most private sources. The viciousness of the slanders and personal denigration also points to MI5; they traditionally "protect" the British state from politicians of the wrong hue by character assassination, and in this case are using their tried and tested methods to murder with words an enemy they have invented for themselves.
And there are precedents. Diana and Hewitt were alleged to have been filmed "at it" by an Army intelligence team which had operated in Northern Ireland, these allegations were made by someone called Jones who had been on the team. His statements were denied by the defence establishment who tried to character-assassinate by describing him as the "Jones twins". Funny how if you tell the truth, then you must be ill, isn't it? Thought only communists behaved like that?
Hewitt later said that he'd been spoken to by someone in the army who revealed the existence of videotapes of him and Diana, and that the tapes would be published if any attempt was made by them to resume their association.