HI Andy
Optimas are good, but they are so B*&%%$y expensive. They also have a smaller capacity than a standard battery of the same size.
I have bought batteries from Tayna.co.uk in the past and very impressed with the service and price. Highly recommended. I didn't pay the extra to guarantee next day delivery, but it arrived the next day non the less.
My advice is buy the biggest heaviest battery that will fit (agricultural/commercial batteries). They are less than half the price of optimas. As long as they last for more than half as long you are ahead.
I managed to fit a 664 (or was it a 644) in my disco (mm to spare), never had any problems and still going strong after 3 years.
Ed