petrol filter

Can anyone please tell me why the petrol filter I ordered does not fit my Diablo (95 model).
I was told by the Eurospares they are the correct type, but my mechanic tells me that the diameter of the screw hole is too small to fit onto my car i.e. a petrol filter of larger diameter needed.
I understand that the Diablo has 2 petrol filters ( one large one small) but the diameters of the connecting part should be the same.

Thanks for your help.

Eric Lee