Hi Nigel
Although it was a long, long time ago when I was marketing my services, I can tell you I called each and every insurer and most major solicitors in Sydney (which is where I was). I pulled whatever con was necessary to get to speak to the claims manager and then asked for an in. I'd get myself invited to submit my CV. I made sure to always get the name of the claims manager even if occasionally I couldn't get to speak to him.
A week after posting the CV, I would call to make sure he read my CV and to discuss my objective. Nine out of ten would say the usual, we'll put it on file (bin it) etc. I'd show my appreciation, make sure to let him know I am very keen and I would call again. A week later I did and I made a nuisance of myself with many of them, but I made the point to them that I would continue my calls until they give me a job (a case) to do. Of course my language was friendly and lighthearted so no one would take offence. After some weeks of hard work and a zillion phone calls and dozens of letters, I got a break. I was kind of like "come and pick up a job because I am getting sick and tired of you".
Within a few more weeks I was able to stop marketing myself and for as long as I stayed in the business, work was coming in by the truck load from most major insurers and large lawyer firms. (a bit immodest there

The point is, you have to be persistent. You have to be confident, be nice and direct. And speak Plain English!
Emails usually go into the junk folder. Easy to do. Everyone emails asking for work. Only a few call and speak. Only through conversation can you make an impression on a client. And then when the letter is sent, it has to be impeccable English.
That's my experience, anyway.
BG
P.S. as for PI agencies I would presume one can take the same approach.
P.S.S. something I used a lot was: "you might not realise it yet, but you need me, you really need an investigator that you can rely on, one that won't cheat you on the invoice, one that knows how to investigate and do it without upsetting people. I am ready for you to give me a go so you can find out just how much you need me". I admit, it depends on your personality. Some people couldn't get away with that
