Comparison with Herdy sheep mattress

Hi how do your mattresses compare with the new Herdy Sheep brand who offer 6000 springs in all natural materials. It's a no turn mattress and I have had experience of a John Lewis 4000 spring mattress which after 6 years was like cotton wool (horrible to sleep on) so I am looking for something comfortable not to firm not to soft once which. Mounds or your body shape suitable for a slatted bed base (recently covered with 3mm plywood). Can you advise please.