For what it's worth, we're taking a different approach on the project I'm currently working on (it's a roughly £3m-£4m project)
Hiring a PM/CM/QS team to run the project and subs
PM/CM/QS team will be paid a fixed fee for the project (we have divided the project into set payment milestones also, to de-risk for client and keep incentives aligned: e.g. demolition, groundworks, etc)
They will assist in hiring the prelims (site manager, welfare, etc) and oversee project on day-to-day basis.
All subs will then be tendered and hired on fixed-fee basis (everything will be itemised and costed by QS)
This means we take on the main contractor risks (and their margin also) - but still keep the tradies working on fixed-fee - which is the only way. Essentially, we are acting as main contractor.
I've been talking to someone recently who ran a big high-end project on open-book with a main contractor (and they've done many projects previously), it's been a disaster. I couldn't recommend this approach or see it working unless the client is highly experienced with many years in the game and has a trusted partner to act as main contractor, and even then I'd be skeptical, the incentives are simply too misaligned (even if they don't appear to be).