Thank you all for your comments so far - very useful.
I'm still ummhhing and aahhhing, trying desperately to justify the change.
Looking in the classifieds, what is the general consensus on age v. cost v. mileage?
For example, 2 cars for sale:
1. RX400h, 2006, £15k, 57k miles (trade)
2. RX400h, 2009, £16k, 65k miles (Car Giant, so ex-company car probably)
All else being roughly equal, which do we think is the better buy?