Tend to agree with KLR. However if you want to compare petrol/diesel you can look on Australian market website for the speci's.
www.holden.com.au/vehicles/series-ii-captiva Their listings are SX, CX & LX (although I think Chev intend to standardiseworldwide to the codes we have here in UK). Chev produce 7 seater in 4cyl in petrol, V6 injected and 2.2 diesel. Just a question of whether you could convince a UK dealer to get a model shipped to UK but 'type recognition' as approved for UK roads would probably be needed. The downside could be parts/spares for a non-standard model as far as UK dealers would be concerned, ie different front grill etc.Only reason petrol option sells in Australia is because petrol there is half the price of ours!
I have 7 seat diesel LTZ (still only 1700 miles on clock)and have to say all round very pleased with it so far in respect of speci v £ cost. See if you can get a dealer to let you have an extended test drive 24/48 hours on diesel - it might change your mind. Only other option is 'sell off' a couple of the kids and the five seater petrol will do!!
Aussie Ed