We've been running events in our church for many number of years with a stated max capacity of 450 (which is admittedly very full). However most events and services have well under 200. Only a few very popular services around Christmas and some external hirer events exceed 250. We've recently had an FRA which has recommended a max capacity of 260 for evacuation purposes. By opening up an external door in our choir vestry the number has been increased to 309. A couple of questions.

1. Are we obliged to restrict numbers in the church based on the FRA given that we feel it may be safe and reasonable to allow larger numbers with other measures in place? 

2. We would like to keep our vestry locked and only open it up as a fire exit when numbers exceed 260. Is it possible to have a temporary fire exit in this way?

If the activity or premises are licensed (any Entertainment Licensing not just alcohol) it is mandatory as the FRA occupancy calculation replaces the mandatory limit previously imposed by the licensing authority.

It should also be deemed mandatory unless a competent person has calculated and can justify a different capacity as overloading premises or having insufficient means of escape would be offences where it was a risk to safety.

Your temporary exit system would usually be fine as long as it wasn't signed as an exit when not in use and was signed when it was.