I'd like to understand this better, but don't know enough about ecology. What does it practically mean to say that heather habitat is "in decline"? What would reverse the decline? Is the 'decline' natural succession, or does it arise from the absence of browsers or grazers from the eco-system? Is it climate-related?
There are some naturally heather-dominated habitats just above the natural tree-line and on areas of deep peat (blanket bog) which are too waterlogged for tree growth. However, most heather-dominated habitats in the UK are anthropogenic in origin - they would once have been wooded and are maintained as open moorland by grazing or burning, or some combination of the two.
Since the second world war the loss of heather-dominated habitats has been attributed to two main causes - afforestation (particularly conifer plantations) and overgrazing by sheep or deer. Moderate grazing can help prevent heather being invaded by woodland or scrub, but overgrazing can lead to it being replaced by grass-dominated habitats. Likewise, heather can benefit from moderate small-scale burning (as found on grouse moors), which results in a more diverse habitat structure than uniform unburnt heather (one of the reasons why managed grouse moors can be a good habitat for other species), but excessive burning (either too frequent or on too large a scale) can have a similar effect to overgrazing - killing off the heather and leading it to be replaced with grassy vegetation.
If grazing and burning stopped, then in theory there would be a natural succession back to woodland- certainly here on Deeside some former moorlands are reverting to native pinewood. That is not generally seen as a problem as it is relatively small scale and the native pinewood is a habitat of value too. However, in many areas, there is often no seed source for trees to regenerate, so what you end up with is tall, dense heather which is neither a 'natural' habitat nor particualrly good for wildlife in comparison to well-managed moorland.
It's estimated that the area of heather moorland declined by 20% between the 1940s and 1980s.