A period of 4 months is set down to comply with conditions which may be inserted by the Building Control Authority in a Regularisation cert. This is not sufficient time and is there a mechanism to extend it?

There is no mechanism to extend. It is an offence to commence work on a building without a Fire Safety Certificate (where required) and the Regularisation Cert is a chance to be in compliance  with the fire safety certification regime. If the works are not carried out within the 4- month period, the certificate will not have effect.