I changed my 12v battery last week as i was getting random warning lights and car not unlocking.The voltage had dropped to 11.4v ,the temperature has not been above 6c for over a month.
Fitting the battery was quite easy and there are many videos on you tube if you are unsure,i found a longer 10mm socket fitted every nut.
I got the replacement battery from Lexus Parts Direct by mail order the next day mine cost £139 + £6 delivery.