Hi

If adding a few extra detectors to an existing fire detection system which does not have a zone plan, is it the installer's responsibility to provide one (or update an existing one if they have one) or is it simply noted on the certificate that it is a non-conformity?

As far as I understand it is the management responsibility to ensure a suitable zone plan, if required, is provided and kept up to date, but I would give the management all the details, if one is required, to enable him/her to carry out their responsibilities. 

Check out 47.2 of Bs 5839-1 2013 especially 47.2.(i)