A bit late in the day, but I managed to get an Exide AGM battery under my seat in a CampervanCo conversion. Terminal positions are slightly different and below the top of the battery but there was plenty of slack in the cables. Important fact it is is a 62Ah battery.

I have the start up rattle on the 3.5 v6 2008, most likely the cam phaser issue. Anyone know of a garage in the south that will do the work? I've spoken to two independent toyota places in Southampton so far, they dont want the work as its too involved I guess.

This sounds more like play in the rear drive shafts CV joints or the drive motor assy itself, as there is nothing random in the E-Four system deciding when to drive the rear motor. From the manual: Start control. The system calculates the total drive torque for the front and rear wheels from the accelerator request drive torque and shift position signal input from the HV control computer, calculates the optimal front and rear drive torque distribution for slippery roads taking into account the front and rear wheel weight distribution, and outputs the calculation result to the HV control computer. This control ensures excellent starting performance on slippery roads. Normal Control. The system calculates the optimal front/rear drive force distribution based on the accelerator request drive torque, shift position signal, wheel speed, steering angle, etc. input from the HV control computer, and outputs a front/rear drive torque distribution signal to the HV control computer to ensure vehicle stability. To excellent fuel efficiency, the rear-wheel drive motor generator is basically inactive and the vehicle runs in front-wheel drive. On slippery roads, the system optimally distributes front/rear drive torque when accelerating, steering, or making tight turns such as U-turns.

I would like to hear from anyone whose E-four Alphard randomly changes from 100% front axle drive to engage the rear axle causing an audible clunk (often heard when accelerating from a standing start). My Alphard has always made a rear axle clunk when first accelerating. I have read this was a common criticism of 1st generation hybrids.

If you are able to swap in a known good battery that will be helpful. Any car battery can likely be rigged up for testing purposes - it doesn't need to be the perfect dimensions, just to make good electrical connections. AGM is better, especially if it's loose in the back (don't want flooded falling over and spilling acid, been there on a previous car...) but if you have another vehicle you could borrow the battery from that for testing, just make sure it's fully charged first and wedge it so it can't fall over. On the Gen2+ hybrids the 12v battery is not cranking the engine so it can actually be quite far gone before it becomes noticeable. All sorts of weird faults can show if the battery is on the way out... a fresh battery and a clear of codes is the first thing to try. One thing I bought recently is a USB charger with battery voltage display: https://www.aliexpress.com/item/1005004622249401.html with the aim to better keep an eye on the battery levels and catch it before it gets too low - the Alphard is much hungrier when locked than my other hybrids.

Thanks as ever Ted. Hope so. The car started a couple of times today but otherwise it's dead. I found a discussion based on a similar problem - it seems to be something electrical and hopefully nothing major.