As it is done on emissions... with the Rx400h, the emissions of 192 (g/km) put the RX400h in band F....which just went up £25 to £190...
So £20 a year saving over the RX300... plus alll the less petrol it uses (in warm weather, at low speed etc) ;)
Still lets be honest, if I spend £35k on a car another £45 a year is not going to make me swap, or influence my decision in any way...