Craig - you've now had TWO people telling you of problems caused by incorrectly specified lower capacity batteries - and those problems were CURED after fitting the correctly specified battery. I really don't understand why you don't listen to and accept that factual advice?!?!?!?!
If your engine blew up, and you had to get a replacement engine - would you fit, say, a 1.6 litre engine (presuming your car is a 2 litre GTI)???? No, so why are you appearing to accept that a lower spec battery is OK?
WHO sold you those incorrect batteries? You need to take them back, and get a FULL refund - the Sale of Goods Act (SOGA) states that a product must be "fit for purpose" - the batteries you were supplied are NOT fit for purpose.
Just stop fannying around, get the correct spec battery (I would personally recommend a Varta E44 - which is a 77Ah with a five year warranty) and your problems will be solved.!