put some battens along the tops of the joists to make it ~200 deep, screwed on occasionally, and the cross joists will keep them from moving, you could get a full 400mm that way.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local