Reading thru all this, can point at one very basic cause and a series of contributory factors and symptoms
The Alarm & Remote sensor.
Tunnie is not completely clear about this, but when you read over a few times, it becomes apparent that his vehicle is being left for a week at a time, sometimes quite a bit longer.....
and then only really getting a little use, before being left again.,
This is largely what happens to our Sintra as well.
What happens is ... when left for a period of several weeks, without being used or touched, the alarm and remote sensor drains the battery a fair bit,but slowly, in a manner that these batteries don't really like.... this is why the manual advises disconnection of the battery is the Vehicle is being "stored long term"
if this goes on for a long time.,.. it can damage the battery permanently, ultimately buckling the plates and any damage like that manifests in reducing it's ability to hold a charge...
then when you charge it, it appears to take a full charge, but 2 days later is flat again....
this process is progressive and cumulative... and it doesn't really reverse ... it can be just a little at a time... , but over time it will eventually become worse and eventually most obviously manifest itself in being unable to hold a charge even over-night.
I found that if the Sintra (Think of it as a V6 omega CDX , mechanically and electrically, sort of... ) is less affected if it's left unlocked and with the alarm un-armed (obviously only practical if you have very secure parking like we do.... when the Sintra is left in front of our garage, in a gated and fenced and locked driveway, with another car between it and any potential exit, it's about as secure as it gets....)
if it's out front though, i have to jump start it after a week.
the big american battery (with side mounted contacts as opposed to top mounted) is expensive.. and I now know what the problem is... so i don;t feel like forking out £80 every time it starts going "off"
Max