We have a big built-in wardrobe but it smells really stale and makes the clothes smell stale too. Any ideas how to solve this?

How about putting tumble dryer sheets (Bounce) on a shelf in side cupboard replace them every week or two should make clothes smell nice!

Leave the doors open as much as possible so fresh air can get in

Is it damp inside due to poor ventilation/heating? Is it on an external wall?

I did suspect it might be damp, but we put some of those boxes with dessicant powder in and no moisture gathered. The cupboard is on an internal wall - we've tried leaving the doors open but again to no effect - as soon as we shut them the smell returns!

try redecorating the inside first and then drill small holes in the doors for ventilations hope this helps