The GWCT have done a stack of work on grey partridge. It all boils down to modern farming and the amount of sprays that now go onto the land. But with good predator control and at least some suitable habitat then they can hold on. The most important factor is don't shoot the ones you have, unless there are good numbers on the ground. Releasing grey partridge has been proven to do little for boosting breeding numbers, the consensus was if you have even one or two pairs of wild birds it is far more benificial to look after them the best you can than release X amount of incubator hatched poults.
I am also 100% that the release of thousands of redlegs has also contributed to the decline of the GP. My little patch of under 200 acres is not ideal partridge habitat, too may woodlands and hedgerow trees, but normally has a couple of coveys of GP and 3 or 4 pairs come spring. A mile or so away the ground is ideal partridge land, rolling chalk downland, few trees, game crops etc, this has a commercial redleg shoot on it but has next to no GP. I would say there must be a disease/ parasite carry over from redlegs to the wild GP.
Bit ironic after the dry spring now we at the peak hatch time for grey partridge the weather has changed to cold and wet!