Thanks for that...couple of questions:

Is it just a case of bigger the better? As I notice just above the Welding cable they do specify Battery Cable in three different sizes:

37071 37/0.71 15 8mm 105 £1.66
37090 37/0.90 25 9.2mm 170 £2.39
61090 61/0.90 40 11.3mm 300 £3.92

Also,

I plan to earth the battery locally in the boot using an earting strap, what size should this be? Bigger the better?

I will earth the existing negative terminals locally under the bonnet and then connect the new positive cable up to the existing terminals under the bonnet [solder them togehter].

Should I add any fuses anywhere? What size?

Does this sound ok? I'm no wiring expert!!