So the rain stopped and I thought the thread was worth updating with an FYI (and title change), as it's now done.
To do it on a 2.0TDI the following are needed:
(This may vary depending on the car, so it is entirely at your own risk - if you're unsure of what you're doing/why or any safety issues, then don't do it and go to a garage).
A new battery (Mine was an 096 AGM to replace the OEM 096 AGM)
A 10mm spanner
A 13mm socket with a long extension (to get to the bottom of the battery).
Petrolium jelly or other similar protective lubricant (probably not required for modern cars)
Pair of suitable nitrile etc gloves.
Suitable Eye/body protection (The battery may be full of acid)
Undo the black negative terminal and place it carefully out of the way (I placed it in a ziplock plastic bag to stop it touching anything).
Undo the red positive teminal and place it carefully out of the way (Again I placed it in a ziplock plastic bag to stop it touching anything)
Remove battery surround (Fabric type thing) by gently pulling upwards and wiggling etc as needed.
If there is a protective cap over the negative terminal, pull the part attached to the battery upwards to remove. (It is in two holes)
Use 13mm socket to undo the bolt at the front side of the battery (At base of battery near positive terminal) and remove the bolt/plate taking care not to drop them into the engine bay.
Slide the old battery forward on the tray until it's released and remove it from the car.
Fit the new battery by placing it on the tray then sliding it into place (May require a wiggle)
Replace the battery plate and bolt, screwing the plate down, so the battery is securely held in place
Wiggle the battery surround (Fabric type thing) back into the correct place.
Clean up the battery connections removing any dirty grease and replace with clean suitable grease. (This is probably not required on modern cars - see later posts)
Refit the red positive lead and do it up (Making sure to keep all tools well clear of the other battery terminal/metal)
Refit the black negative lead and do it up (Making sure to keep all tools well clear of the other battery terminal/metal)
Push the negative protective cap back into the holes by the negative terminal to cover it
Close the lid of the battery surround and make sure everything is securly in place.
Next you need to code the battery, so will need VCDS or another suitable tool.
Again if you don't know what you're doing, just pay a garage to supply, fit and code the new battery.
Code the new battery using VCDS under modules / CAN gateway (19) / Adaptations (10) and then selected the correct values for your new battery in the categories below:
IDE03256-MAS06105-Battery adaptation-Rated battery capacity,
IDE03256-MAS06106-Battery adaptation-Battery technology,
IDE03256-MAS06107-Battery adaptation-Battery manufacturer,
IDE03256-MAS06108-Battery adaptation-Battery Serial Number,
My OLD Battery
Capacity 68 Ah
Technology Fleece
Manufacturer JCB
Battery Serial Number 1111111111
My NEW Battery
Capacity 70 Ah
Technology FLEECE
Manufacturer YBX
Battery Serial Number 1111111112 (incremented by 1)
Check your new battery specs, as it may well be different to mine, for example EFB instead of AGM, different capacity/manufacturer etc) and make sure you pick the correct values for your new battery.
I then took a very short drive for about 100 yards to sort out the ABS sensors all being implausable and warnings to go out.
The tyre pressure warning system complained it had lost all values, so they were checked and that reset.
Finally I auto-scanned the car, checked for faults and cleared any fault codes related to the low battery.
All seems fine and it started instantly so I'll keep an eye on it over the next few days to make sure the battery has solved the original issue.
Regarding the old battery and why voltage (with no load) isn't really useful in deciding if it's dead, it's still at a constant 12.7V without load.
Stuck it back on the CTEK now to see how long it takes to get back to float charge (Shouldn't be long if it's healthy) and then I'll get it load tested in 24 hours to see what's left and if that is the likely candidate.