The vast majority of the many self-penned CVs I've read was utterly terrible.
I disagree, Vince. The vast majority of the many self-penned CVs I've read was utterly terrible. Perhaps the online writing services houses churn out bland, templated CVs but a proper writer will create a bespoke and punchy document that will stand out and be professional.
It's a bit of a weird one I guess as I am sure there are services out there that will write you a cracking CV but just beware of the "churn houses" as described in my post. As I said at least half of the 300 cv's were near on identical in terms of layout, qualifications, length of sections etc. As a employer/decision maker I found it incredible that people seemingly couldn't be bothered to spend the time required to write a CV. Personally I have several CV's tuned for different roles, I have a managers cv, a proper techie CV and a cut down version that sort of includes both aspects. Depending on the role depends on how a gear the CV. To be fair though I have never had to use it as I have never had a proper interview, I have always been approached when not looking for work by people ive previously worked with. It probably helps that I worked for a small bespoke company that employed around 100 FD's who were put into FTSE and AIM listed companies on an interim basis. I used a CV writing service to do my CV, create a cover letter and do my LinkedIn profile last year.
I was having interviews every week for months and the reason why I have the job I have now.
So it was worth it for me but I did give them my old CV to work from.
They got mentioned last time this topic appeared, turned out they just use a template and fill it with pointless waffle/generic phrases etc...
But I had a 45min phone consultation with them, sent them my old CV. After 2 days they improved it and that's when all the interviews started flooding in.
I doubt very much that a changed CV in itself caused interviews to suddenly "flood in", I suspect that if your old CV had been dormant on some recruiting portals then *any* update to it (assuming it wasn't dire) would trigger contacts from recruiters.
Likewise if you actively applied for roles after getting a new CV then...
Its not getting so many hits in google this time but when they were mentioned before then you could search for the below waffle and get a bunch of linked in profiles:
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A highly experienced, commercially astute business leader with a wealth of high end professional expertise and technical knowledge, an individual with strong leadership and consensus building skills with expert man management abilities, and a proven track record of delivering results during every stage of the business life-cycle.
A self-starter and highly organised individual able to work well at all levels, with the ability to solve problems and make decisions enabling the efficient and timely completion of work. A professional business leader focussed on delivering first rate service on all projects from inception to completion through effective management and team collaboration
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It doesn't really matter if they've changed the waffle, the point is just that generic waffle like that is such a waste of time. Everyone can proclaim to be a "self starter" etc.. and have an "ability to solve problems" etc.. who cares? It's totally meaningless.

You didn’t really address the points other than just commenting that I’m being negative - how did the apparent flood of job interviews get triggered?
You were actively applying?
I mean I don’t doubt that if your CV was dire before then it can help but it seems like the same can be achieved by just using a template yourself if really needed.

...Ive already explained 
were
I'd agree with Vince tbh... these services seem to just use a standard template and copy and paste generic statements/buzzwords etc...
In that sentence, "majority" is a singular collective noun so "was" was correct to use.