I had an offer from a private sector client with a strong ethical reputation in Manchester.
Everything looked great until I saw the paperwork.
As well as a good main Agency contract from a Hopefully Normal agency, there was an extra doc to sign called <ClientName>_Amendments_<date_in_2012>.doc.
This had provisions saying 'Clause x.y of main contract shall not apply' - that was the substitution clause - and something like 'contractors shall be subject to the supervision and control of the hiring manager'.
Queried immediately and this was definitely a *client* stipulation, admittedly not from my hiring manager but from the top of the organisation. No amount of pressure from me or the hiring manager, who was extraordinarily keen and was reasonably high up their food chain, could get it changed.
According to the agent, despite them having dozens of contractors on site and having placed a lot more there down the years - I was the first person to have ever queried it, and they were all working those terms.
Fortunately I had another offer and although I wanted this one... I walked. (Yes I know there is a *chance* the engagement could still have been outside IR35 but on balance it was way outside the risk profile MyCo's Directors choose to operate under).
Moral of the story: read every bit of detail in your contracts and cross-reference clause numbers and exceptions!