Cheers Monkey

I think that should do for now and I will give it a go when I get back. So in theory checking the voltage across the battery terminals whilst out of the car ie my old OEM on should be the same - 12V.
I take it that a good battery will hold its 12V if not used and out of the car pretty much continually. So in a couple of months time when I get home I can get the multimeter on it to see if I wasted my money or not.
Pip