http://quaife.co.uk/alfa-romeo-front-wheel-drive-sequential/

Does anyone know if this quaife is a straight fit into a six speed coupe please.

No - the Coupe uses a Quaife QDH6E-6

I'm thinking that the gear kit on the link is compatible with the QDH6E-6
No?

I recall that the Quaife in my Coupe was an Alfa GTA unit, but it had to be slightly modified to fit - can't remember how or why

The first difference that I can remember straight away is that Alfa Romeo 156/147 GTA use their ABS sensors for the speedometer reading.

Fiat Coupe uses some kind of plastic gearwheel attached to the diff. So there are 2 notches/holes for this gearwheel to be locked. I think there aren't such holes in the GTA diff because they don't need it.

Although this is not a major problem if you use the car only for track days...

You can use Alfa 156/GT Q2 diff without a problem, it fits right into 6 and 5 speed diffbox as well. You need to purchase a gear speed sensor which needs to be put on the differential OE PN fiat 7783658.