You're right, there are currently no other publishing companies in the UK operating that scenario. You could try talking to some of the big ad agencies who currently outsource their production personnel to get their view on how it works. Their rates would probably be 10 times yours but you might be able to get some interesting pointers. I have a list of many of these agencies somewhere if you need it.
Whilst I appreciate that you are looking for a contractors point of view, you must, at all times, consider your Board's interests. Would you employ a temp agency to fill all your secretarial posts on an on-going permanent basis? I doubt it. Why do it for production pre press?
People with the kind of skills you are looking for would bite your hand off at the moment to be offered a permanent role in your company (I know who you are btw :wink

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The other alternative, and forgive me if this sounds a bit obvious, is to train your current staff. The training cost would predominantly be a one-off cost with minimal on-going cost. It will work out an awful lot cheaper in the long term.
Management in publishing companies, I believe, have always underestimated the potential of their production personnel. They also overestimate how long it may take to learn the skills required to achieve the pre press quality you are looking for.
Sorry to digress from what your original topic, but before you put any proposals forward you have to consider it from your Board's point of view.