I'd still suspect the battery first. It will be interesting to see what Halfords have to say. None of the steps or tests you've conducted so far are definitive when it comes to battery condition. What matters as much as cell voltage off load is voltage at cranking current. It's possible to meet the former and fail the latter, on a battery which is at end of life. And whilst it may still be in warranty, letting lead acid batteries fully discharge and sit like that can, in my experience, be a kiss of death for them.
Another thought is; how are you jump starting? I've also attempted to jump an D5 XC90 with a heavily discharged battery before and hit similar symptoms to yourself. Moving the jump leads from the under bonnet terminals, to directly onto the battery in the boot resolved the issue. I suspect that the former is just a path of too high resistance (corrosion on the bolts, diameter of cable), if a very significant current contribution from the donor vehicle is required.
__________________
2007 XC90 V8 Sport
|