She bought herself a new car back when he was gambling but before he confessed to her, and he was very graceless about her having done so.
And then, back in April 2006, we got
"Shula tells [Caroline] about Alistair’s outburst over the car, and how Daniel caught the full force of it. She can’t take Alistair’s mood swings, but blames herself; she should have seen it coming, and now there’s nothing left between them. And even worse, she’s depriving Daniel of a father for the second time. "
In other words, it’s not as if this were a new disillusionment with him.
Disappointed we didn’t get to find out the colour, not convinced there is much requite for a convertible in the West of England though, also I am not sure it would have the umpfh to tow a horsebox. That said, why should Alastair care if she buys a new car? I didn’t get the feeling he was particularly stingy with money when they were together - the huntmistress conversations for instance.
Since he was going to stump up for that - because she said she couldn’t afford it - he might feel somewhat aggrieved that she suddenly has money to fritter on sublimating what would effectively be Kenton’s vicarious mid-life crisis.