I think a 235 has a bigger rolling radius, so you may encounter a bit of an issue there, with you being so low, Robin.
An ET48 on an 8" rim, will mean it stick out 9mm further than your Monzas. 3mm for the change in offset and 6mm for the 1/4 of an inch added to either side of the barrel.
However, a 225 will only be affected by slightly changing the position of the tyrewall closest to the rim, as it gets towards the tread, it'll still be a 225's width, remember. So in reality you're still only 3mm further out, increasing to 9mm the further you compress the suspension. Make sense?
This size, offset & tyre combo is perfect IMO.