Many thanks for all your help, it is much appreciated. Apart from test driving a 2006 GS450h when it was new, my only experience of Lexus cars has been with nearly new petrols and diesels, I've never owned a hybrid version before. This particular car I'm interested in is at a car auction so unfortunately I won't be able to get it inspected before buying it. It definitely needs to be a lot cheaper to make the risk worthwhile.
One check I was thinking of making was the mpg readout on the car's computer - am I right in thinking that if it is low, say in the mid 20s mpg, the battery could have problems because the car is mainly running on petrol and that if the mpg is higher, say mid 30s mpg, the battery is likely to be in good health as the car will be using the battery a fair bit. I'm down in Devon so I'm not sure if there is anyone vaguely local who could recondition the battery for me.
Many thanks again :)