The vRS Fabia's tyre speed rating is w on the factory fit conti's. Does this mean that when I get tyres for my new 17" wheels that I have to stick with w or will v rating be o.k., as it is well in excess of the cars top speed (it's not a 2 litre model ). Will there be any legal or insurance issues with the lower rating.

Technically, you should stick with W rating for insurance purposes. Practically, as they still exceed the max speed of the vehicle you should be OK

But dont forget the load rating also.

But dont forget the load rating also.

I'm looking at Pzero Nero's at 215/40/17 for the Fabia. They are Z rated which I guess is why the sidewall seems stronger and makes them pretty good with the Fabia's heavy front end. What part is the load rating and have the Nero's got a high enough one?

Yes they do, IIRC they are 91 or 94. Standard ones are 89 I tthink