No doubt as a reminder of previous current drain issues, I have a battery isolator switch - without a working key fob, is there a way to avoid or minimise any risk of the alarm going off when unlocking and reconnecting the battery?

Open the door, open the bonnet, close the door and then reconnect the battery. As long as the status when the battery is reconnected (doors open or closed, doors locked or unlocked, etc) is the same as when it was disconnected, you shouldn't have a problem.

If the car is going to be left for any length of time, open the bonnet, then close and lock all the doors, disconnect the battery and close the bonnet. When you want to reconnect, open the drivers door (which, with the battery disconnected will be the only one that will unlock), open the bonnet, close the door and lock it, then reconnect the battery.

How much current drain do you have? Do you have the BeCM SID with the current drain diagnostic flowchart, if not, it's attached.
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Info you need is from page 10 onwards.

As usual, that’s a great help - that was my procedure when disconnecting, so hopefully I won’t be annoying the neighbours.

Haven’t had a chance to investigate current drain yet; have a new battery waiting to go on, so will monitor then - thanks.

Disconnect the alarm sounder...