I have just picked up a second-hand V1 and I am looking for a carrycot for a newborn and I’m not sure what is compatible with the v1.
Well, the first thing I'll suggest is to look at classifieds. The carrycot for the Duet V1 model was the older type (as opposed to the convertible type that was introduced from the second generation which requires a set of adaptors), and as an older model, it is not sold anywhere. Pre-loved, used ones (if you'll be able to find any) would be the best fit
This is how the bassinet looks like (so you know what you're searching for):
If you won't be able to find the right carrycot, I would go for a soft, insert carrycot or cocoon that would be a universal solution to your problem. From Hauck 2-in-1 Carrycot Convertible to Footmuff or a Diono Newborn Pod to Phil&Teds Cocoon (Soft Carrycot), just check what's available for your double stroller... and, if possible, try in a baby shop, if you want to be sure about the fit. But you should be fine either way - these are made to be pretty versatile.