Problem with Vi Spring Herald Superb Mattresses

I bought a Vi Spring Herald Superb mattress with Medium support which turned out to be faulty - basically it collapsed in the middle and offered no support at all, as well being extremely uncomfortable. Vi Spring agreed there was a manufacturing fault and replaced it on a like for like basis. The same thing happened with the replacement mattress and now Vi Spring have offered me another replacement. Vi Spring say problems with these mattresses are very unusual (they would wouldn't they), but having had a year of uncomfortable and expensive mattresses I'm wondering whether to risk trying a third mattress, or to try and ask them to recommend a different mattress, or a refund even.
Any comments or advice would be gratefully received.