Sorry, I should have come back with an update:
We had a rather large meeting with managers & stakeholders from all sides. I presented my view and the designers stated theirs. I was accused of 'working against business by my managers'. I explained I was doing my job!!! It became clear that the problem could be avoided if the scaffolding erection (and the later dismantling) were scheduled out of hours, the 58 hours or so between 19:30hrs on the Friday, to 06:00hrs on the Monday. There was plenty of contingency safety margin available, but I did accept the costs would be vastly more expensive.
The Project Manager whinged on about budgets and overtime working ....., and we were coming to a point where it looked like a stalemate position was inevitable. I suggested we bring the local fire safety enforcement team in to act as an arbitrator, with their decision (when it was received in writing) being final. All parties agreed. The meeting ended before lunch.
The next day at 10:00, even before I had contacted to local fire safety inspectors, the Project Manager contacted me and said a decision has been made to go with the out of hours build idea. I knew he was up against the ropes and said I would only sign the proposal off if the contractors agreed to pay substantial penalty fees if they overran the 06:00 finish deadline. (Frankly, I didn't believe they were committed to the programme).
3 days later and all parties had signed up to the out of hours install and penalty fees and the build has already started. We have had a hiccup with neighbours complaining about the noise at 3am! but that is not my problem
1-0 to me I reckon