Further to my earlier post where I considered fitting a second battery wired in parellel with the original, I have now done this and have attached the photo below for reference.
These batteries are each 30Ah with a CCA of 350. The 2 together therefore amount to 60Ah with a total CCA of 700.
Needles to say, the engine turns over very quickly and starts within a few seconds of the first press of the starter button.
I know this is extra weight to carry but having been unable to start the car recently on the original (now 3 year old) battery, I am looking for reliability (if this is possible anyway with the earlier M3Ws) and if my 0 - 60 time or top speed has been reduced slightly because of the extra weight, it will not worry me.
its best policy to draw current diagonally from paralled batteries to offset the effect of the minute extra contact resistance, which will carry greater significance when a high starter current is drawn. This allows power to be drawn equally from each battery. In this case taking the positive or negative feed from the opposite battery would achieve this.