As Papaindigo said, it'll probably be a group 34R. It'll need a fair whack (probably about 800CCA) to start the diesel. I'd go for another optima redtop or an Odyssey - both are very good, both are sealed and I think both are AGM - certainly the Odyssey is. Neither is cheap - probably about the £200 mark. Have a look at the ads of any UK LR/4wd magazine, there are quite a few dealers who sell both, but just tell them what model you want, rather than the vehicle you want it for.
There are those who say that the quality of optima has gone down in the last few years - I don't know about that because the one the dealer put in my CRD in 2006 is still going strong, but learning from the experience of others is why I come on this site! If that is the case, though, the Odyssey might be a better bet! Also have a look at Exide - I think they do a high CCA sealed battery as well but I don't know what they are like - I THINK they are stocked by Extreme4x4 - but again - have a look at the ads. Let us know how you get on.
I've just had a look - try
www.tayna.co.uk. They stock Optima, Odyssey and Exide (the Maxxima looks to be the one to go for) - proces around the £240 mark.