It was an absolute nightmare to do but I'm in a place where I'm happy with the procedures and it's been tested with fire drills (albeit during quieter times so far). I ended up creating three different assembly points, with multiple fire wardens sweeping areas on their route out to one of those assembly points, and a building controller role acting as the central point for comms with the fire wardens and control panel. Each assembly point would have at least one fire warden at it with communication between them all via radio. Each role has its own physical pack, with hi viz, instructions, radio etc for them to take with them, which seems to have helped them.
All the staff have had training as fire wardens since I posted last, and some of them have also been trained with the evac chair. I'm currently trying to establish why or license says we can have a maximum of 7 wheelchairs on site at any one time when we only have 1 evac chair (7 is an entirely unreasonable number).
All in all, this procedure seems to be working well. We had a visit from the Fire Service recently who were happy with it.
The main issue that remains is staffing levels, since in order for this system to work we need more staff to be on site, either on rota or working in the small office (we have high levels of staff WFH). This is a management issue and I've made it clear to those in charge that correct levels of staffing are an absolute must.
This has been a big learning exercise for me and an interesting one!
from me and a thank you for returning with your update. 
