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.
Ben & Tim I will go out to the workshop this afternoon and take a few photos. After working out what size fitted my battery box I then went to my local Supplier and when through there stocks to find one that fitted ( not much room where my batteries are located) hence using 2 12V batteries
Originally Allards were fitted with two 6 Volt Batteries connected in series to give 12 Volts, Although the starters are 6 Volts they seem to handle 12V OK.
My recommendation is to fit two 12Volts and connect in parallel. Varta make a battery which fits the battery box. When the engine plays up you cannot have too much cranking power and two 12’s make jump starting much easier. Note. As well as the batteries being earthed the engine should also be earthed.