Hi Gareth & Cliff,
Many thanks for your replies. Just an update on things, the reason why I was trying to take the battery off was due to the fact that I'd tried a jump start yesterday but had no joy (albeit I only tried for a couple of minutes) plus I was too far away from my flat to be able to charge it whilst attached, hence why I thought I might be able to just whip it off and do a main charge indoors. In the past I've flattened the battery by leaving the dashcam plugged in overnight during cold periods but normally I could open the car with the key fob, the ignition lights would come on, but just get the flickering when you try to fire up the engine cold, but this time I had none of that.
Good news is that I was able to do a prolonged jump start this morning (ie leaving it charging for about 10 minutes) and although I had disconnected and reconnected the battery a few times last night, apart from the date and time, nothing else had visibly reset (e.g radio) so I'm back in business.
Interestingly what I did notice is that initially when I realised I'd drained the battery, I had zero response from the car (had to unlock drivers door manually, no dash display or interior light and completely dead when ignition key turned) which made me think the battery was knackered this time, especially when having tried a quick jump start yesterday it was still fully unresponsive. This morning however, as soon as I reconnected to the battery terminals with a 2nd jump start attempt, the alarm immediately went off (this time with flashing indicators), so disabled that via the key fob and when I re-open the drivers door to my surprise the mileage and time was now showing on the dash plus the interior light came on and I could turn on the radio, none of which was possible when I did the jump start yesterday (so behaving like it has done in the past when the battery was flat from leaving the dashcam plugged in overnight)
It's almost as though it had provisionally gone into some kind of power hibernation that it only was woken out of after I'd done the multiple disconnect / reconnect last night that had triggered the alarm, then jumped it for a second time this morning (the only way I can describe it!)
Very confusing, but at least I'm all good now and also learned that getting zero response from a flat battery doesn't necessarily mean it's completely knackered!
Now it's time to get the new battery sent back that arrived this morning :') !