The othoptera order, grasshopesrs and crickets, present a tricky identification challenge. Like the common field grasshopper, the meadow grasshopper (Chorthippus parallelus) is very common but, unlike the 'field', it has green around the face and thorax. The abdomen tends to be stripy brown and quite often has tinges of orange and I have even found one bright purple (this often occurs in the females I believe).
The meadow grasshopper, as it name implies, can be found on almost any grassland habitat but is especially common where the grass is moist. Just after the entrance to Powerstock Common there is some rough, moist, grassy scrub and as you walk through it you see 'clouds' of these jumping out of the way of your path. This species is also common on the Dorset heaths too.
If you have good enough hearing then you may catch their 'song', like a sewing machine in three second bursts and repeated every ten to fifteen seconds gradually getting louder. You should be able to find both the 'meadow' and the 'field' from July thriugh until the end of October and possibly in to November if the weather remains mild.