An interesting thing that isn't being made clear is WHY journalists would use these 'search agents' like Steve Whittamore etc. Sherborne's implication is that they were always a force for evil illegally accessing information, but that's just not true.
Back in the days before extensive wifi, journalists would travel around the country on stories with laptops but no internet access. These agents could be rung up and would be able to provide information from their extensive digital databases (sometimes from Land Registry, directory enquiries, electronic birth/marriage/death records, and enormous advertising databases which are put together based on those boxes you tick on forms to allow your information to be used). All of those are totally legal, as long as you pay the enormous fees to access them.
The really good agents could jigsaw together information and cross-ref sources to work out where people lived, who their families were and where they lived, and any contact information. It's a real investigative skill.
That might seem unsavoury, but not illegal. I suspect most of the information being obtained was along these lines. And the reason the Mail/MoS had the most prolific usage is that they were a huge operation and had journalists travelling around the country/the world all the time because they had the budget.