The sheet is only on one portion of the roof (about 25%) and has a big gap at the bottom so hopefully condensation won't be an issue - I could put some breathing holes in it as well and the loft space seems very dry generally all year round.
Hi
Part of my roof has no felt under the tiles which means a lot of dust and dirt comes in. To help keep things stored up there a bit cleaner I've tacked polythene sheeting (http://www.wickes.co.uk/Polythene-Vapour-Barrier/invt/153230) onto the underside of the eaves where felt is missing...