I noticed you can now get retro fit AeroTwin wiper blades.

https://www.autobulbsdirect.co.uk/home.php?cat=779

They seem to come in metric sizes and the closest to the 21" and 19" required for the fabia is a 530mm and 475mm pack. These would probably bit a little short. The other option is to go slightly longer say 550mm and 500mm but too long would definitely be a bad move.

Has anyone got any experience of which size fits best?

I've got the 21"/19" set and they are fine.

They don't seem to actually come in 21 / 19 though. Is it the 530 / 475 you've got? Which is pretty close to a 21/19 equivalent.

Renault Megane 2 Aero wipers are a perfect fit

You want Bosch part number AR533S for the Fabia.

thanks all.