How does a deal involving a part-exchange and a factory order usually work. With factory-build Citigo lead times in the Porsche territory of several months, what happens if you order a factory-build but have a part-exchange car to put down as your deposit? With lead times of nearly 4 months, does the dealer take your current car away when you place the order?

No, they give you a price for the part exchange valid for when your Citigo arrives.

So you keep driving your old car until the day you turn up for the new one.

....and you drive, and park, it very carefully so that nothing nasty happens to it. In the past, I've had two bodywork "mishaps" on two different cars while waiting for new ones to arrive. Apparently, it's quite a common thing to happen!

can confirm also sometimes if it is a long lead time they may ask to revalue vehicle or value it closer to date, had one of mine where they said they could not guarantee trade-in price all the way to delivery so we valued it nearer the time and they gave me a good price

I'd ask the dealer.

Before my gf got a new polo we were looking at a citigo with a 5 month wait and the dealer gave us a PX price which he said would be valid if we bought the car and collected it 5 months later.