The narrator, David Bellamy, was a fixture on British nature programmes. Not so much now but in the 80's and 90's his beard-muffled vocal stylings were commonplace. Controlled burning is kind of fascinating, the 2 country gents commissioned this video that is now in the BFI.
Heather is kept young and vigorous by controlled burning. If left unburned, it eventually grows long and lank, reducing its nutritional value.
Burning is carried out by moor keepers between the Autumn and Spring when small sections are burned carefully on a rotational cycle, which can be as little as 7 years where there is very vigorous growth or as long as 25 years where growth is slow. These fires are very different to uncontrolled emergencies and are restricted by law to the period between 1st October and 10th April.
...For example, controlled burning will take place when the heather cover is dry, the peat wet and the wind light but constant. This ensures that the fire moves steadily over the peat, burning the plant but leaving the peat bed relatively cool.
The heather roots are left undamaged and the whole process 'shocks' the heather seed lying in the ground into germinating quickly. Heather can also be cut with a mower, especially in places where lighting a fire would be too dangerous, although mowing is less effective, more expensive and only possible on certain ground (for example uneven surfaces with rocks are unsuitable).
-via The Moorland Association