One possible solution is to use insulated ducting:

https://www.i-sells.co.uk/product_in...oducts_id=2465

I believe it is mandatory on new installations under part L. The above company are quite good for having a comprehensive stock of various ducting and their helpline is particularly good at helping you get the right stuff, the bloke I spoke to was very good even though I was only spending approx. £60 with them. Alternatively if your duct is running along the top of the joists just feed your existing insulation over the top of it. I am not sure if it will entirely solve the condensation issue but will surely limit it..

Richard