Steve, a word of caution, I once had a "Dash Rattle" on a 306 DTS, I was convinced it was behind the glovebox, stripped it and found nothing, it was only on tickover if stood still. I eventually traced it to the engine, a clip had broken and a cable was knocking on the bulkhead.
Is does sound like its coming from behind the dash but thanks for the advice i will definately check the engine bay out first.
Cheers