It sounds like whoever installed the previous heat alarm used grey as the neutral conductor, and black as the interlink. I an ideal world these cores should have been sleeved to indicate their purpose!
The easiest way to confirm what wire is doing what would be to use a multimeter to indicate the voltage coming through each cable. If you have access to one of these and measure between the brown and grey wires, you should have 230 volts.
Obviously great care should be taken when getting a reading, and if you're not 100% confident to do this I'd suggest calling someone in to give you some help.