Nearly right Fred!
A generic 40mm lowering kit fitted to a G40 will only lower it around 15mm over standard. Like you said a G40 is already 25mm lower than a normal Coupe.
If you're looking for a really good off the shelf suspension kit, the H&R Cup Kit is 60mm lower at the front with the option of 40 or 60 mm lower at the back by way of the spacer provided. So you can reckon on it being about 30mm or so lower than a standard old G40 setup.
It's certainly reckoned to be one of the best O.E. type "kits" available. It retains enough ground clearance to be useable, it's obviously firmer and remains much flatter through corners, yet it's still compliant enough for some comfort.
Your broken rear spring isn't uncommon on these Polos Fred.
Hope this helps mate.
Cheers,
Richard.