Thanks. I've used the short-term parking many times, but this time I'll be away for longer, so I'm considering using the long-term parking which is 140 baht a day, instead of 250 baht.
The airport information makes no mention of whether you need to make a reservation or can just turn up, or how and when payment is made. There is also little information about how to access the lots and whether they are related to the terminal you're flying from, the accessibility of the shuttle, how frequent it is, and how long to allow between parking and checking in.
Was hoping someone might use it regularly, but perhaps you're right and a taxi would be better!