November was expected to be a poor month - and it was. Fiat UK's overall sales were down 42% compared with last November. However, if you look at year-to-date, they're still doing OK, as JKDesign says. December is predicted to be better, and the VAT cut is likely to encourage those who've been holding off.
Although I don't have figures handy, 500 sales are still strong from what I hear, and as long as that continues, there's no reason to move to reduced production.
Certainly as far as the Abarth goes, we know that Abarth can sell at least 200 of them tomorrow, if they bothered to actually bring them into the country, and that's an argument for speeding up production, not reducing it.
I've not heard that the launch will be delayed until March. From what I can remember, it was always going to be around February time.