A friend of mine is thinking off registering as a Ltd company with Number Mill, has anybody ever had any dealings with them? and are they any good?, their website is not very plain and transparent.
iirc Number mill was setup by Milestone, now staffline?
So afaik its their own MSC / payroll Co. But it was setup in the wake of the MSC scandal. So potentially they could be playing by the rules
I’d put money on it that HMRC will have “investigated” Number Mills procedures. They, like many of these schemes that are available will be ducking and diving around the current legislation.
My advice would be to get your own Accountant, roughly £500 per year, and stay clear of these schemes. HMRC are attempting to collect taxes from people who have been shafted by such schemes. Oh, and HMRC aren’t exactly squeaky clean in all of this, they took over 4 years to establish if Think Accounting, (another mickey mouse scheme) were invoicing on their clients behalf, a fact that could have been established in a week or so,and indeed was known by HMRC back in 2011, but then they waited and waited and waited and then pounced and landed the Drivers with massive back dated NI/tax bills because they had been entrapped by the MSC legislation.
Shark infested waters out there.
As advised above, do your own thing, there is absolutely no reason to be using these sharks 
If you are “advised” by an agency to use their preferred supplier, just move on to the next one, until you find the one that is honest and trustworthy 
There are a few out there…honest
the only scheme I know who have been fully investigated and held a full consultation with hmrc is nova (they had to change a lot because of this).
HMRC are investigating all of the schemes due to the MSC Legislation which was created in 2007 and is only now being enforced.
nova had theirs done last year they only offer a couple of options now because of this - just fyi I think anyone considering doing it this way would be far better off getting an accountant and doing it for yourself.
Stanley Mitchell:
If you are “advised” by an agency to use their preferred supplier, just move on to the next one, until you find the one that is honest and trustworthy
iirc you are automatically enrolled in NM when you sign up with MS/SL unless you specify otherwise. I encountered NM a year ago when the anniversary of my accounting period passed & parked my company post bleak season to switch to a caring roll at home, however in April I took a couple of shifts via MS on the proviso it was PAYE. So when the paperwork was setup to switch SE to PAYE I was informed NM would handle the payroll aspect
Staffline, which up until recently were known as Milestone(March 2016), pay the Ltd Co Driver’s through the Driver Exchange 
Why not keep it simple?