Having issues with my battery

So I've been having issues with battery drain for just over a year or so now.
It would normally take between 3 and 4 days of being stood to drain but I fitted an isolator and all was good, well as good as it could be
But last week it started dropping over an 8 hour period, only by a volt or so but enough to not start.
Needless to say it's become a concern as I dare not go to far in case I can start it up, can anyone offer up any ideas?
I don't know if it's anything to do with the long term drain getting worse but I'm just wondering if it has or its something else entirely.

Cheers

What about the voltage with engine running ? ... and what kind of disco ?

Sorry it's an 02 plate td5 D2, it's charging at around 14.29-14.35

You'll have to measure drain on the engine bay fuses, use the procedure from the video then report on which fuse you get the big value

You sure the battery itself is in top working order and not been on it's way out??

In my experience, when a car battery has died, it's never the same again...

I thought my RR had a drain but it didn't.... Was just the battery slowly coming to it's death

The battery is only a year old and the initial drain was before it was replaced

I take it you have connected an ammeter between the bat and its connector on one side to check the amount of current drain once it is just sitting there, "turned off"?

Every 2 hours in an 8 hour period it dropped just over half a volt

R U sure all interior lights, in the glove box etc are going off? Nothing else plugged into a system not via the ignition switch first position? No wires chafed through with a positive feed earthing to the chassis or bodywork? Did you follow SF's thing?

Even a year old battery can be duff, one cell goes down and takes power from the other five,
Disconnect battery and fully charge her, leave her for 24 hours then check it’s voltage,
If it’s below 13volts it’s the likely suspect, very often alternators can consume power feeding back through the diodes,
So disconnect the heavy brown lead and see if you are still loosing power overnight,

A battery that's been left off charge overnight should settle down to around 12.8v at best if there is no draw on it