Hive Receiver loose cable

hi folks, long time lurker, first time poster. I've recently had a Hive hub with various components (heating, switches, bulbs). All working great until a few days ago when the heating wouldn't go on. Unfortunately I've found a grey cable loose out of one of the terminals. I'm hoping someone can help me determine which terminal it's supposed to go in please.

Looking at the length of cable, I'm sure it's either terminal 1 or 2. N and L are both fine, as well as 3 and I don't think terminal 4 is used.

I've attached a photo (hopefully successfully) to show what I'm talking about.

Terminal 1.

Thanks for the swift response. I had seen this schematic but thought it best to seek advice just to be on the safe side.

Very wise.

And that's it working now! Very much appreciate the response. Thanks.