I have just bought some porche 996 alloys because they were a massive bargain, will they fit my fabia or are they simply too big?

Sizes and widths would help.

You will be looking at around 250 quid to get some good quality PCD adapters. without knowing the wheel dimensions it's hard to say if they will fit under the arches, or what size adapters you would need.

Assuming they are genuine, and you have purchased a complete car set then you may well struggle to get the 996 rears onto the Fabia.

 

Front alloy wheels: 17x7" ET55

Rear alloy wheels: 17x9" ET55

The 18" variant is even worse.

Front alloy wheels: 18x7.5" ET50

W

Rear alloy wheels: 18x10" ET65