Thought my reply post had been submitted OK, Ben – but it seems this medium is getting a bit clunky… I attached a photo of my battery location and the system doesn’t appear to have taken it.
In a nutshell, it depends on the shape and dimensions of your battery box. From Halfords I got the biggest that would fit under the bonnet, just forward of the engine bulkhead – a single item, costing £92. Some folk take the opportunity to relocate the relatively heavy thing further back, to assist with weight distribution.
Tim.