The simple answer is not that simple. Like all health and safety issues it’s all about looking at your processes, your hazards and the associated risks. The deciding how they apply to you organisation and implementing suitable control measures. Sadly there is no one simple answer/solution.

Yes you do have a duty of care, so something needs to be done, but so do the self employed fitters who would be classed as contractors, they also have a duty of care (Section 3 of the HSWA74). So any approach to health and safety (including toolbox talks) needs to be a joint approach.

Weekly toolbox talks seem to be a bit excessive, monthly sessions are more reasonable I think. Deliver them too often and I think that there is a risk of getting to "burn out point" too soon. The trouble is, if it all goes Pete Tonge can you prove that you did what a reasonable employer should have done at the time? Toolbox talks are not the only solution. Neither for that matter is getting the contractor to totally sign off the health and safety issues as being their responsibility carte blanche.

You mentioned working for a local authority. That may actually be your driving force behind the need to start delivering tool box talks and undertaking site inspections (combining health, safety, environment and quality issues). Many LA’s are now filtering out contractors such as yours that do not have ISO 9001, 14001 and a robust health and safety management system in place. My experience is that the requirement to maintain an ISO or OHSAS standard is sadly often the only reason why tool box talks and site visits are undertaken.