Why are they disappearing?

The reasons aren’t clear but more intensive agriculture – with larger fields and the loss hedgerows and permanent grassland – is likely to have played a role. The use of pesticides too will reduce the amount of prey available. Badgers are a natural predator of hedgehogs and hedgehogs actively avoid sites where badgers are present. When the habitat provides sufficient cover and good foraging opportunities, predation by badgers is not necessarily a problem for hedgehogs, but in poor habitats, it might have a big impact.

Something we really need to keep in mind even in our own gardens .. My Wildes are living in the bit of garden waste we have behind the old shed at the top of the garden x

I agree the increase in chemical use. Putting out slug pellets for instance. As well as us living in a chemical world now.
The lack of hedgerows and perhaps people putting out milk or bread. Both of which hedgehogs will eat but should not eat doesn't help.

Adverts like the ones on TV at the moment do not help .x

Certainly don't